[custom channels]: finish sweeping of custom channel HTLCs (part 6/5)

[guggero]

The custom channel saga is almost complete, the only missing part is to finish some of the HTLC sweeping code for custom channels.

[dstadulis]

Concepts considered during previous design sessions:
During forced HTLC sweeps, create a sharedGotoChain call to enhance lnd's decision to go to chain or not.

[dstadulis]

Related PRs:
#9194
lightninglabs/taproot-assets#1154