BOLT 12 support for bLIP-52/LSPS2

[TheBlueMatt]

[tnull]

We already have LSPS1 support (thought the corresponding bLIP PR is still pending lightning/blips#59), so this is only regarding LSPS2 / bLIP-52.

To support BOLT12 in conjunction with LSPS2 we will require a way to include the intercept SCID in the BOLT12 flow.