[feature]: Experimental HTLC Endorsement

[carlaKC]

As described on the mailing list, we'd like to use an experimental TLV (endorsed / 65555) in update_add_htlc to "dry-run" propagation of HTLC endorsement. This would be an incredibly valuable way to validate that "steady-state" payment flows are unaffected by the addition of endorsement/reputation and observe the proposed reputation algorithm's behavior in the wild.

Part 1: Passive Relay of Endorsement Field

The first step will be to get the experimental field relayed within the network (on an opt-in or opt-out basis):

• If LND receives TLV 65555 on the incoming update_add_htlc:
  • Set the outgoing update_add_htlc's TLV 65555 to the value on the incoming link.
• Otherwise:
  • Set the outgoing update_add_htlc's TLV 65555 to []byte.

Endorsement is a chain of signals which is completely lost if even a single node along the route does not set it. Updating nodes to relay the TLV would be incredibly valuable, as that preserves the chain of signals and allows a very small subset of experimenting nodes to use this value.

Part 2: Active Experimentation

Participants that volunteer to participate in the experiment will start to set these values:

1. Senders: set endorsed = 1 for some portion of their payments.
2. Forwarders: set outgoing endorsed to 0 or 1 on the outgoing update_add_htlc (based on the current spec's instructions) -> requires [feature]: Add Outgoing Custom TLVs to HTLC Interceptor #7841.