Trampoline forwarding [2]: HTLCSource variant refactors

[a-mpch]

This PR is on top of #3983 is part of the split-up of #3976

Main focus of this PR is to add a TrampolineForward variant to HTLCSource enum, then we extract most logic to be re-used and then implement the places where we require it.

After this PR only left is the implementation of finding path and forward the HTLCs and adding tests for all reamaining "to do" statements

[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[bot]]

Codecov Report

:x: Patch coverage is 85.77348% with 103 lines in your changes missing coverage. Please review. :white_check_mark: Project coverage is 88.72%. Comparing base (381416a) to head (9268af2). :warning: Report is 110 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channelmanager.rs	81.33%	48 Missing and 8 partials :warning:
lightning/src/ln/onion_payment.rs	59.67%	25 Missing :warning:
lightning/src/ln/blinded_payment_tests.rs	95.49%	12 Missing and 4 partials :warning:
lightning/src/ln/onion_utils.rs	0.00%	6 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4027      +/-   ##
==========================================
- Coverage   88.94%   88.72%   -0.22%     
==========================================
  Files         174      175       +1     
  Lines      124575   128464    +3889     
  Branches   124575   128464    +3889     
==========================================
+ Hits       110797   113979    +3182     
- Misses      11278    11919     +641     
- Partials     2500     2566      +66     

Flag	Coverage Δ
fuzzing	21.69% <24.10%> (-0.50%)	:arrow_down:
tests	88.55% <85.77%> (-0.21%)	:arrow_down:

Flags with carried forward coverage won't be shown. Click here to find out more.

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

:rocket: New features to boost your workflow:

• :snowflake: Test Analytics: Detect flaky tests, report on failures, and find test suite problems.