Add HOP promotion: runtime-side

[franciscoaguirre]

In order to support promotion to Bulletin in HOP, we need both node-side and runtime-side changes.
This PR adds the runtime-side changes in order to test them before we move Bulletin Polkadot to fellowships.
The node-side changes are in paritytech/polkadot-sdk#11484.

How promotion works

The node regularly scans for entries about to expire and for each, it calls the runtime API HopPromotionAPI to get the promote() extrinsic it needs from pallet-hop-promotion. This extrinsic is authorized only if it comes from the local transaction pool, meaning it can't be sent from the outside world. It can only be used by collators building a block to promote. Since the extrinsic is also given a low priority from the runtime, it'll only be included in a block if it has space available after all user extrinsics.

sp-hop was created here mimicking the same crate on the sdk until we merge that PR.

Approach

The promote() extrinsic:

• Uses general transactions via FRAME's #[pallet::authorize] macro
• No signature is required. Calls are submitted by collators without signatures
• No fees. Protocol-level operation not submittable by regular users
• Priority 0, the minimum. To make sure all user transactions take precedence over these ones.
• Local-only, TransactionSource::External gets rejected.
• Not propagated to other nodes
• Longevity 5: probably too little, can be increased. It will however be constantly re-created if needed by the node long-running task.
• Delegates to pallet_transaction_storage::Pallet::do_store(data, Blake2b256, RAW_CODEC)

Companion: sdk

The companion to this PR also declares the runtime API and uses that to submit these promote() extrinsics.