feat(cawg_identity): Add SignerPayload struct

[scouten-adobe]

Split off from #644.

[codecov]

Codecov Report

Attention: Patch coverage is 94.44444% with 1 line in your changes missing coverage. Please review.

Project coverage is 80.09%. Comparing base (ca8e18b) to head (89b8325).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
..._identity/src/identity_assertion/signer_payload.rs	87.50%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #817   +/-   ##
=======================================
  Coverage   80.08%   80.09%           
=======================================
  Files         118      119    +1     
  Lines       31285    31299   +14     
=======================================
+ Hits        25055    25069   +14     
  Misses       6230     6230           

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