itests: Assert payment+invoice HTLC custom wire records are persisted, returned

[dstadulis]

Given lightningnetwork/lnd#8769 has been merged, this issue can be started.

As discussed in lightninglabs/taproot-assets#1091, litd should update integration tests to

• Assert the lnd changes to persist payment+invoice HTLC custom wire records and return them in RPCs hold true.

Specifically, itest should validate:

• (maintainer edit: seems this functionality has been confirmed in [feature]: persist payment+invoice HTLC custom wire records and return them in RPCs taproot-assets#1091) SendPaymentResponse_AcceptedSellOrder response is returned with the correct assets amount
• (maintainer edit: seems this functionality has been confirmed in [feature]: persist payment+invoice HTLC custom wire records and return them in RPCs taproot-assets#1091) Custom data fields are added to the lnrpc.InvoiceHtlc and lnrpc.HtlcAttempt messages
• Wire-level custom message data attached to HTLCs in invoices and payments is persisted
• Functional purity / existing functionality doesn't break

[guggero]

Fixed by #879.