Limit payment path length based on `payment_metadata`, custom TLVs, etc.

[valentinewallace]

Currently the maximum payment path length is hardcoded in the router to 19. However, the presence of payment_metadata, custom TLVs, and/or blinded paths may mean that the actual limit is much shorter. Account for these extra onion fields when calculating and setting the maximum path length for use in pathfinding.

Closes #2201.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 97.88732% with 12 lines in your changes are missing coverage. Please review.

Project coverage is 89.85%. Comparing base (da7a916) to head (3cc673b). Report is 4 commits behind head on main.

Files	Patch %	Lines
lightning/src/routing/router.rs	77.14%	8 Missing :warning:
lightning/src/ln/max_payment_path_len_tests.rs	99.22%	2 Missing :warning:
lightning/src/ln/outbound_payment.rs	91.66%	1 Missing and 1 partial :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3026      +/-   ##
==========================================
+ Coverage   89.79%   89.85%   +0.06%     
==========================================
  Files         116      117       +1     
  Lines       96466    97278     +812     
  Branches    96466    97278     +812     
==========================================
+ Hits        86619    87413     +794     
- Misses       7290     7314      +24     
+ Partials     2557     2551       -6     

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[valentinewallace]

I found some more edge cases around how get_route counts hops as part of a path, so feel free to hold off on review there.

[valentinewallace]

Rebased to (hopefully) fix CI. This should be good for review.

[valentinewallace]

Feedback should be addressed. Also this needs a rebase so let me know when I can do that.

[valentinewallace]

Squashed with 1 new fixup.

[valentinewallace]

Squashed.