impl(auth): simplify signer and override IAM endpoint impersonated credentials

[alvarowolfx]

Towards #4186

[codecov]

Codecov Report

❌ Patch coverage is 96.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 95.05%. Comparing base (8e336e1) to head (6cd2de9).
⚠️ Report is 23 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/impersonated.rs	96.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4738      +/-   ##
==========================================
- Coverage   95.07%   95.05%   -0.02%     
==========================================
  Files         198      198              
  Lines        7587     7589       +2     
==========================================
+ Hits         7213     7214       +1     
- Misses        374      375       +1     

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