Serialization for Vec<BlindedMessagePath>

[joostjager]

Required for serializing async payments blinded message paths as part of the ldk-node api.

[ldk-reviews-bot]

👋 Thanks for assigning @tnull 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 88.75%. Comparing base (04b9a17) to head (8db8813).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4057      +/-   ##
==========================================
- Coverage   88.77%   88.75%   -0.03%     
==========================================
  Files         176      176              
  Lines      129375   129375              
  Branches   129375   129375              
==========================================
- Hits       114855   114821      -34     
- Misses      11915    11944      +29     
- Partials     2605     2610       +5     

Flag	Coverage Δ
fuzzing	22.02% <ø> (ø)
tests	88.58% <ø> (-0.03%)	⬇️

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[tnull]

Simple enough, one nit

[ldk-reviews-bot]

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[TheBlueMatt]

Please include context in the commit message so that we know why we did something from looking at git history, not just in the PR.