[custom channels]: in itest assert payment and invoice HTLCs show custom data

[guggero]

Depends on #848.

With this commit we sporadically assert in our itests that the outgoing HTLCs for a payment and the incoming HTLCs of an invoice show the custom channel data JSON with the actual asset amounts.

Fixes #860.

Example invoice

assets_test.go:862: Asset invoice: {
            "memo":  "this is an asset invoice over 100000 units",
            "r_preimage":  "0b47e516136f867eb8f578ec44e240d4935643050ad47583b14a6c26ffbb20e2",
            "r_hash":  "cb502385800559c07812597ee863556fca2c981451668c57a582ad998ebd034d",
            "value":  "1718035",
            "value_msat":  "1718035941",
            "settled":  true,
            "creation_date":  "1729615613",
            "settle_date":  "1729615613",
            "payment_request":  "lnbcrt17180359410p1pn304happ5edgz8pvqq4vuq7qjt9lwsc64dl9zexq529ngc4a9s2kenr4aqdxsdzyw35xjueqd9ejqctwypshxum9wssxjmnkda5kxefqdamx2u3qxycrqvpsxqs82mnfw3escqzzsxqzfvrzjq0qpeu7ra3xl2pxgrfkdlj0s0yvp99xvmltzw0xsezz6srunx6esdazhpj8h8twt3sqqqqlgqqqqqqgq2qsp5dpm8vjvzn3uyw6xsdc4swc6s74zll7ucpakdwldcjg2jd7rtt8aq9qxpqysgq8jualj4mpv9x0wd906hpe2a93mq40klfznrs0rfqrn0qdc5s3a6sekl6l8uvvjayes36a6s7fck8k7853ykg5yklnj9hn6sfjqrgslsqmyczep",
            "description_hash":  "",
            "expiry":  "300",
            "fallback_addr":  "",
            "cltv_expiry":  "80",
            "route_hints":  [
                {
                    "hop_hints":  [
                        {
                            "node_id":  "03c01cf3c3ec4df504c81a6cdfc9f079181294ccdfd6273cd0c885a80f9336b306",
                            "chan_id":  "17606555078488869772",
                            "fee_base_msat":  1000,
                            "fee_proportional_millionths":  1,
                            "cltv_expiry_delta":  80
                        }
                    ]
                }
            ],
            "private":  true,
            "add_index":  "5",
            "settle_index":  "5",
            "amt_paid":  "1718035941",
            "amt_paid_sat":  "1718035",
            "amt_paid_msat":  "1718035941",
            "state":  "SETTLED",
            "htlcs":  [
                {
                    "chan_id":  "537661186179072",
                    "htlc_index":  "18",
                    "amt_msat":  "1718035941",
                    "accept_height":  494,
                    "accept_time":  "1729615613",
                    "resolve_time":  "1729615613",
                    "expiry_height":  577,
                    "state":  "SETTLED",
                    "custom_records":  {
                        "65536":  "012c0020bb9eedadf7e7985639fbd60b1c6ea97b5eb9b4ae55f4f63685ba480a42e5db070108000000000001869f",
                        "65538":  "a812a383f14908081a3b289c55b9e77aac5caab6cbf00345f68fa397327597ac",
                        "65543":  "006c016a0020bb9eedadf7e7985639fbd60b1c6ea97b5eb9b4ae55f4f63685ba480a42e5db0701407386a43b1722319acd6b32e42a83e0bf1835f7a45992a6a7aaa52d141c553877731d2748c06f68fb5c468c3e40fac62909d4859ee690cc95bd5983f56926a77e020400000083"
                    },
                    "mpp_total_amt_msat":  "1718035941",
                    "amp":  null,
                    "custom_channel_data":  "7b2262616c616e636573223a5b7b2261737365745f6964223a2262623965656461646637653739383536333966626436306231633665613937623565623962346165353566346636333638356261343830613432653564623037222c22616d6f756e74223a39393939397d5d2c227266715f6964223a2261383132613338336631343930383038316133623238396335356239653737616163356361616236636266303033343566363866613339373332373539376163227d"
                }
            ],
            "features":  {
                "8":  {
                    "name":  "tlv-onion",
                    "is_required":  true,
                    "is_known":  true
                },
                "14":  {
                    "name":  "payment-addr",
                    "is_required":  true,
                    "is_known":  true
                },
                "17":  {
                    "name":  "multi-path-payments",
                    "is_required":  false,
                    "is_known":  true
                },
                "25":  {
                    "name":  "route-blinding",
                    "is_required":  false,
                    "is_known":  true
                }
            },
            "is_keysend":  false,
            "payment_addr":  "68767649829c784768d06e2b076350f545fffb980f6cd77db8921526f86b59fa",
            "is_amp":  false,
            "amp_invoice_state":  {},
            "is_blinded":  false,
            "blinded_path_config":  null
        }

Where the custom channel data evaluate to (would be decoded automatically in lncli, the above is directly from the itests):

{
  "balances": [
    {
      "asset_id": "bb9eedadf7e7985639fbd60b1c6ea97b5eb9b4ae55f4f63685ba480a42e5db07",
      "amount": 99999
    }
  ],
  "rfq_id": "a812a383f14908081a3b289c55b9e77aac5caab6cbf00345f68fa397327597ac"
}

Example payment

assets_test.go:904: Asset payment: {
            "payment_hash":  "cb502385800559c07812597ee863556fca2c981451668c57a582ad998ebd034d",
            "value":  "1718035",
            "creation_date":  "1729615613",
            "fee":  "0",
            "payment_preimage":  "0b47e516136f867eb8f578ec44e240d4935643050ad47583b14a6c26ffbb20e2",
            "value_sat":  "1718035",
            "value_msat":  "1718035941",
            "payment_request":  "lnbcrt17180359410p1pn304happ5edgz8pvqq4vuq7qjt9lwsc64dl9zexq529ngc4a9s2kenr4aqdxsdzyw35xjueqd9ejqctwypshxum9wssxjmnkda5kxefqdamx2u3qxycrqvpsxqs82mnfw3escqzzsxqzfvrzjq0qpeu7ra3xl2pxgrfkdlj0s0yvp99xvmltzw0xsezz6srunx6esdazhpj8h8twt3sqqqqlgqqqqqqgq2qsp5dpm8vjvzn3uyw6xsdc4swc6s74zll7ucpakdwldcjg2jd7rtt8aq9qxpqysgq8jualj4mpv9x0wd906hpe2a93mq40klfznrs0rfqrn0qdc5s3a6sekl6l8uvvjayes36a6s7fck8k7853ykg5yklnj9hn6sfjqrgslsqmyczep",
            "status":  "SUCCEEDED",
            "fee_sat":  "0",
            "fee_msat":  "0",
            "creation_time_ns":  "1729615613604251687",
            "htlcs":  [
                {
                    "attempt_id":  "19",
                    "status":  "SUCCEEDED",
                    "route":  {
                        "total_time_lock":  577,
                        "total_fees":  "0",
                        "total_amt":  "1718035",
                        "hops":  [
                            {
                                "chan_id":  "537661186179072",
                                "chan_capacity":  "100000",
                                "amt_to_forward":  "1718035",
                                "fee":  "0",
                                "expiry":  577,
                                "amt_to_forward_msat":  "1718035941",
                                "fee_msat":  "0",
                                "pub_key":  "031bb2310f97d81b3219bc3c72a96400beb95a3287e43c1f7ed762e9bf4bce24e1",
                                "tlv_payload":  true,
                                "mpp_record":  {
                                    "payment_addr":  "68767649829c784768d06e2b076350f545fffb980f6cd77db8921526f86b59fa",
                                    "total_amt_msat":  "1718035941"
                                },
                                "amp_record":  null,
                                "custom_records":  {},
                                "metadata":  "",
                                "blinding_point":  "",
                                "encrypted_data":  "",
                                "total_amt_msat":  "0"
                            }
                        ],
                        "total_fees_msat":  "0",
                        "total_amt_msat":  "1718035941",
                        "first_hop_amount_msat":  "354000",
                        "custom_channel_data":  "7b2262616c616e636573223a5b7b2261737365745f6964223a2262623965656461646637653739383536333966626436306231633665613937623565623962346165353566346636333638356261343830613432653564623037222c22616d6f756e74223a39393939397d5d2c227266715f6964223a2261383132613338336631343930383038316133623238396335356239653737616163356361616236636266303033343566363866613339373332373539376163227d"
                    },
                    "attempt_time_ns":  "1729615613623285861",
                    "resolve_time_ns":  "1729615613844891774",
                    "failure":  null,
                    "preimage":  "0b47e516136f867eb8f578ec44e240d4935643050ad47583b14a6c26ffbb20e2"
                }
            ],
            "payment_index":  "7",
            "failure_reason":  "FAILURE_REASON_NONE",
            "first_hop_custom_records":  {
                "65536":  "00",
                "65538":  "a812a383f14908081a3b289c55b9e77aac5caab6cbf00345f68fa397327597ac"
            }
        }

Decoded:

{
  "balances": [
    {
      "asset_id": "bb9eedadf7e7985639fbd60b1c6ea97b5eb9b4ae55f4f63685ba480a42e5db07",
      "amount": 99999
    }
  ],
  "rfq_id": "a812a383f14908081a3b289c55b9e77aac5caab6cbf00345f68fa397327597ac"
}

[gijswijs]

I couldn't even find a nit, and I tried, so LGTM! 🎉