tracking: remaining cleanup to address before merging `0-19-staging` branch into master

[guggero]

This issue tracks all cleanup that is necessary to address before we can merge the 0-19-staging branch into master.

• Add more RPCs for scid aliases #8509 (comment) (collision and range of custom local SCID aliases)
• Add more RPCs for scid aliases #8509 (comment) (error handling for unknown custom local SCID alias)
• Add more RPCs for scid aliases #8509 (comment) (collision detection and validation of base SCID)
• Enhance update_add_htlc with remote peer's custom records #8660 (comment) (implement and test HTLC custom record persistence)
• Enhance update_add_htlc with remote peer's custom records #8660 (comment) (unit test coverage of all added custom record functionality)
• Enhance update_add_htlc with remote peer's custom records #8660 (comment) (persistence of custom records across restarts when sending payments)
• [4/5]: lnwallet: add new AuxSigner interface to mirror SigPool  #8632 (comment) (mock implementation for aux components and basic unit test coverage)
• [6/5]: invoice+rpc: add exit hop InvoiceAcceptor sub-systems and RPC calls #8771 (comment) (add more fields to invoicesrpc.HtlcModifier
  • Marked as done because no additional fields were required for the custom channel functionality.
• revert itest change in 4f2c75f to properly test amount modification
  • Need to use invoice acceptor to be able to pull this off, so create invoice for amount X, actually send X*2 (which'll be in the MPP amount to expect), intercept HTLC to modify it back to X then use invoice acceptor to settle with just X and ignore MPP total amount)
• [custom channels 5/5]: merge custom channel staging branch into master #8960 (comment) (remove hardcoded sweep output amount)