Fuzz StaticInvoice

[elnosh]

As part of #2298, add fuzzing for StaticInvoice message.

[ldk-reviews-bot]

👋 Thanks for assigning @valentinewallace 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 88.90%. Comparing base (6a4169d) to head (0854152).
⚠️ Report is 17 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #4005   +/-   ##
=======================================
  Coverage   88.90%   88.90%           
=======================================
  Files         174      174           
  Lines      125114   125114           
  Branches   125114   125114           
=======================================
+ Hits       111234   111235    +1     
- Misses      11364    11365    +1     
+ Partials     2516     2514    -2     

Flag	Coverage Δ
fuzzing	22.13% <ø> (+<0.01%)	⬆️
tests	88.73% <ø> (+<0.01%)	⬆️

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[TheBlueMatt]

Thanks!