LSPS2: Add support for ongoing proportional fees

[aylagreystone]

Adds support for ongoing proportional fee proposal for bLIP52 lightning/blips#63

Still waiting for more feedback on the bLIP change but having it implemented in at least one implementation seems to be a requirement for merge so preparing one for LDK.

[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 89.82%. Comparing base (b8673f3) to head (0045de2).
Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3846      +/-   ##
==========================================
- Coverage   89.88%   89.82%   -0.06%     
==========================================
  Files         162      162              
  Lines      130415   130420       +5     
  Branches   130415   130420       +5     
==========================================
- Hits       117217   117145      -72     
- Misses      10509    10576      +67     
- Partials     2689     2699      +10     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

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