Ensure transaction transmissibility due to HTLC signature-creation requirements by limiting transaction's asset UTXO input count

[dstadulis]

Limiting UTXOs in Funding Output:

Reason: Ensure conformance with message-size limits on protocol level
Cause:
- Each HTLC may reference multiple asset UTXOs from funding output
- Each referenced input needs a signature, increasing transmission data
Alternative Solutions:
- Implement SIGHASH_NOINPUT on protocol layer
- Allows sending second-level signatures alongside HTLCs
- Requires stable pkScript for next commitment (achieved with per-commitment-secret)
- Limit total number of UTXOs within funding output
- Asset ID is not the limiting factor, but rather total UTXO count

[Original commentary]

[dstadulis]

due to wire message limit