feat(cawg_identity): Split CredentialHolder into sync and async versions

[scouten-adobe]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 86.19529% with 41 lines in your changes missing coverage. Please review.

Project coverage is 79.13%. Comparing base (0b9f122) to head (9f8bebb).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ity/src/builder/async_identity_assertion_signer.rs	70.09%	32 Missing ⚠️
..._identity/src/builder/identity_assertion_signer.rs	69.56%	7 Missing ⚠️
...identity/src/builder/identity_assertion_builder.rs	98.48%	1 Missing ⚠️
sdk/src/signer.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #891      +/-   ##
==========================================
+ Coverage   78.96%   79.13%   +0.16%     
==========================================
  Files         143      144       +1     
  Lines       33978    34172     +194     
==========================================
+ Hits        26831    27041     +210     
+ Misses       7147     7131      -16     

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