Cache commitment point on ExternalHTLCClaim to drop a signer call

[alecchendev]

Small standalone change that will also help make things simpler for async signing.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.66%. Comparing base (df01208) to head (4c1638c).
Report is 51 commits behind head on main.

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Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3096      +/-   ##
==========================================
+ Coverage   89.90%   91.66%   +1.75%     
==========================================
  Files         117      119       +2     
  Lines       97105   112257   +15152     
  Branches    97105   112257   +15152     
==========================================
+ Hits        87303   102901   +15598     
+ Misses       7243     7085     -158     
+ Partials     2559     2271     -288     

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