feat(cawg_identity): Introduce IdentityAssertionSigner

[scouten-adobe]

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[codecov]

Codecov Report

Attention: Patch coverage is 47.27273% with 29 lines in your changes missing coverage. Please review.

Project coverage is 78.99%. Comparing base (ce78e14) to head (c23b1c1).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._identity/src/builder/identity_assertion_signer.rs	47.27%	29 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #827      +/-   ##
==========================================
- Coverage   79.04%   78.99%   -0.06%     
==========================================
  Files         119      120       +1     
  Lines       31158    31213      +55     
==========================================
+ Hits        24630    24656      +26     
- Misses       6528     6557      +29     

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