Rework ChannelManager::funding_transaction_signed

[wpaulino]

Previously, we'd emit a FundingTransactionReadyForSigning event once the initial commitment_signed is exchanged for a splicing/dual-funding attempt and require users to call back with their signed inputs using ChannelManager::funding_transaction_signed. While this approach worked in practice, it prevents us from abandoning a splice if we cannot or no longer wish to sign as the splice has already been committed to by this point.

This commit reworks the API such that this is now possible. After exchanging tx_complete, we will no longer immediately send our initial commitment_signed. We will now emit the
FundingTransactionReadyForSigning event and wait for the user to call back before releasing both our initial commitment_signed and our tx_signatures. As a result, the event is now persisted, as there is only one possible path in which it is generated. Note that we continue to only emit the event if a local contribution to negotiated transaction was made.

Future work will expose a cancellation API such that we can abandon splice attempts safely (we can just force close the channel with dual-funding).

[ldk-reviews-bot]

👋 Thanks for assigning @jkczyz as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

❌ Patch coverage is 74.57143% with 89 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.31%. Comparing base (de384ff) to head (135605f).

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	69.59%	39 Missing and 6 partials ⚠️
lightning/src/ln/channelmanager.rs	67.59%	29 Missing and 6 partials ⚠️
lightning/src/ln/splicing_tests.rs	90.78%	2 Missing and 5 partials ⚠️
lightning/src/events/mod.rs	87.50%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4257      +/-   ##
==========================================
- Coverage   89.33%   89.31%   -0.03%     
==========================================
  Files         180      180              
  Lines      139042   139030      -12     
  Branches   139042   139030      -12     
==========================================
- Hits       124219   124172      -47     
- Misses      12196    12217      +21     
- Partials     2627     2641      +14     

Flag	Coverage Δ
fuzzing	35.96% <5.38%> (-0.01%)	⬇️
tests	88.68% <74.57%> (-0.03%)	⬇️

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