Async Send Simple Hold

[joostjager]

Simplified variant for sending of async payments that doesn't use trampoline. The sender LSP receives an HTLC from the sender indicating that the HTLC should be held. Sender LSP then sends out HeldHtlcAvailable to the receiver LSP. Receiver LSP replies with ReleaseHeldHtlc. Upon receiving this onion message, sender LSP release the held htlc and lets it continue along the sender-determined path.

[ldk-reviews-bot]

👋 Hi! I see this is a draft PR.
I'll wait to assign reviewers until you mark it as ready for review.
Just convert it out of draft status when you're ready for review!

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.93%. Comparing base (39e8d7d) to head (aa9e84b).
⚠️ Report is 311 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3957      +/-   ##
==========================================
- Coverage   88.93%   88.93%   -0.01%     
==========================================
  Files         174      174              
  Lines      123876   123905      +29     
  Branches   123876   123905      +29     
==========================================
+ Hits       110173   110198      +25     
- Misses      11250    11253       +3     
- Partials     2453     2454       +1     

Flag	Coverage Δ
fuzzing	22.19% <65.00%> (+0.01%)	⬆️
tests	88.76% <100.00%> (-0.01%)	⬇️

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[joostjager]

Superseded by #4045