[Custom Transactions] Add TxBuilder trait, support fixed additional outputs

[tankyleo]

hi @TheBlueMatt on this branch here i make an attempt at an API that supports "fixed additional outputs" on commitment transactions.

I place today's 330 sat anchors in that bucket, but the API should allow for custom ones.

The API should also let people customize all the existing outputs on the commitment transaction.

Still need to think about any remaining TODOs for additional outputs that are only present for some commitments (eg. DLC outputs), depending on how we'd like to prioritize these.

One TODO that remains is abstracting away the calls to chan_utils::commit_and_htlc_tx_fees_sat in channel dust exposure calculations.

[ldk-reviews-bot]

👋 Thanks for assigning @TheBlueMatt 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

Attention: Patch coverage is 95.10309% with 19 lines in your changes missing coverage. Please review.

Project coverage is 90.37%. Comparing base (5316257) to head (14b83a3).
Report is 49 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	92.40%	5 Missing and 14 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3775      +/-   ##
==========================================
+ Coverage   89.53%   90.37%   +0.84%     
==========================================
  Files         157      160       +3     
  Lines      125310   133578    +8268     
  Branches   125310   133578    +8268     
==========================================
+ Hits       112192   120721    +8529     
+ Misses      10429    10231     -198     
+ Partials     2689     2626      -63     

☔ 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.