Support async signing of interactive-tx initial commitment signatures

[wpaulino]

This commit allows for an async signer to immediately return upon a call
to `EcdsaChannelSigner::sign_counterparty_commitment` for the initial
commitment signatures of an interactively funded transaction, such that
they can call back in via `ChannelManager::signer_unblocked` once the
signatures are ready. This is done for both splices and dual-funded
channels, though note that the latter still require more work to be
integrated. Since `tx_signatures` must be sent only after exchanging
`commitment_signed`, we make sure to hold them back if they're ready to
be sent until our `commitment_signed` is also ready.

Depends on #4336.

[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 76.96737% with 120 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.08%. Comparing base (8cdc86a) to head (9933019).
⚠️ Report is 15 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	75.24%	66 Missing and 9 partials ⚠️
lightning/src/ln/channelmanager.rs	79.62%	35 Missing and 9 partials ⚠️
lightning/src/ln/interactivetxs.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4355      +/-   ##
==========================================
- Coverage   86.53%   86.08%   -0.46%     
==========================================
  Files         158      156       -2     
  Lines      103190   102620     -570     
  Branches   103190   102620     -570     
==========================================
- Hits        89300    88340     -960     
- Misses      11469    11775     +306     
- Partials     2421     2505      +84     

Flag	Coverage Δ
fuzzing	?
tests	86.08% <76.96%> (+0.24%)	⬆️

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[TheBlueMatt]

Cool! Easy enough. We should ask claude to add async signing to the chanmon_consistency fuzzer as a followup.

[TheBlueMatt]

Marked this as fixing #3489 and #2088 though we should probably see if we need to address #3109 (review) as well.

[wpaulino]

Marked this as fixing #3489 and #2088

There's one more case for dual-funding that still needs to be addressed -- the first_per_commitment_point in open_channel2.

[TheBlueMatt]

Sure but we don't actually do dual-funding yet, so that's just a part of doing dual-funding now :)