Compact `BlindedPath` representation for onion messaging

[jkczyz]

BOLT 12 defines a compact representation for BlindedPaths, which uses short_channel_id in place of next_node_id for use in an OnionMessage packet's hop data: https://github.com/lightning/bolts/pull/798/commits/ba2b95a98a415a24c27685a34648c1f64468a4bf

Additionally, the blinded path's introduction node can use a short channel id with a direction byte: https://github.com/lightning/bolts/pull/798/commits/3db064e4cb943885ff5557181ae655d03ac8ce79

TODO:

• [ ] Create a BlindedPath that uses a directed scid introduction node #3011
• [x] Parse a BlindedPath that uses a directed scid introduction node
• [x] Pay a BlindedPath that uses a directed scid introduction node
• [x] Resolve a directed scid from a BlindedPath introduction node
• [ ] Create BlindedHop::encrypted_payload that use an scid as the next hop #3011
• [x] Parse ForwardTlvs that use an scid as the next hop
• [x] Resolve an scid from a PeeledOnion::Forward

Related issues:

• [x] Create an OnionMessage using a BlindedPath that has the sender as introduction node correctly #2951
• [ ] Pay using a BlindedPath that has the payer as the introduction node