add HTLCs to ledger channel update protocol

[NiloCK]

This is draft work toward including HTLC transfers as part of the base consensus_channel.

Strongest caveat: there is no on-chain implementation to enforce any of these data types.

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[geoknee]

This is looking good so far @NiloCK !

I think we probably want:

• [ ] an ADR to accompany these changes, explaining how LN-compatible multi hop payments can be incorporated into go-nitro. You've probably got the germ of that in Notion already.
• [ ] some unit test for the functionality you built so far
• [ ] eventually an integration test involving any changes to the go-nitro API (etc). We could develop that as a feature branch and merge it all in one go

[NiloCK]

:+1:

Will probably shelve this until after the hackathon.