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]

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[codecov]

Codecov Report

❌ Patch coverage is 86.27451% with 14 lines in your changes missing coverage. Please review.
✅ Project coverage is 86.03%. Comparing base (dde2c82) to head (ee74209).
⚠️ Report is 13 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	83.33%	11 Missing and 3 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4355      +/-   ##
==========================================
+ Coverage   86.00%   86.03%   +0.02%     
==========================================
  Files         156      156              
  Lines      102857   102909      +52     
  Branches   102857   102909      +52     
==========================================
+ Hits        88465    88538      +73     
+ Misses      11883    11865      -18     
+ Partials     2509     2506       -3     

Flag	Coverage Δ
tests	86.03% <86.27%> (+0.02%)	⬆️

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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 :)

[ldk-reviews-bot]

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[wpaulino]

Had to rebase due to small conflict after #4311