[Custom Transactions] Define the `TxBuilder` trait

[tankyleo]

    [Custom Transactions] Define the `TxBuilder` trait
    
    This trait will give users of LDK the ability to create arbitrary
    commitment transactions given a snapshot of the Lightning state machine.
    
    For now, we only make it available for internal use. Once further
    work elsewhere in LDK is complete, this trait will become public.

[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!

[tankyleo]

@TheBlueMatt I believe we are at a good point where we can define this API internally. Still much work to do before making this public, but I thought we could get this done now let me know.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 86.97917% with 25 lines in your changes missing coverage. Please review.

Project coverage is 89.38%. Comparing base (559a784) to head (3a2f8c9). Report is 11 commits behind head on main.

Files with missing lines	Patch %	Lines
lightning/src/ln/channel.rs	53.70%	25 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3761      +/-   ##
==========================================
+ Coverage   89.14%   89.38%   +0.24%     
==========================================
  Files         157      158       +1     
  Lines      123997   126674    +2677     
  Branches   123997   126674    +2677     
==========================================
+ Hits       110531   113227    +2696     
- Misses      10786    10818      +32     
+ Partials     2680     2629      -51     

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[tankyleo]

API is not stable yet I am still making big changes to it. Will reopen soon.