Liquid: Covenant Refunds for overpaid amounts

[kilrau]

We want to enable users that sent too much L-BTC in a Liquid-to-Lightning or Liquid-to-Chain swap to be able to claim refunds for the overpaid amount themselves using covenants. Refund address will be provided in the refund step, just as-is.

[kilrau]

Just leaving a comment here that I manually refunded a couple of overpaid liquid swaps recently and logs show that some of the users actually tried to refund themselves via our web app (but it failed with Not creating partial signature for refund of Chain Swap ABC: it is not eligible {"level":"verbose"}). Meaning we can expect most users that accidentally overpaid to try refund themselves, worst case we just point them at it - still a lot less work than refunding manually.

So this feature is significant for reducing support volume.