feat(auth): add retry loop for generate_id_token

[alvarowolfx]

Towards #4459

[gemini-code-assist]

Code Review

This pull request introduces a GenerateIdTokenClient with an internal retry loop to handle token generation. However, the review feedback points out that this creates nested retry loops because the token provider is already wrapped in an outer retry mechanism. This nested structure can lead to retry storms and ignores user-configured retry policies. It is recommended to remove the inner retry loop and let the outer provider handle retries.

[codecov]

Codecov Report

❌ Patch coverage is 96.87500% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.04%. Comparing base (2f59820) to head (be4318e).
⚠️ Report is 10 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/idtoken/impersonated.rs	96.87%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main    #6090    +/-   ##
========================================
  Coverage   97.04%   97.04%            
========================================
  Files         253      253            
  Lines       63682    63792   +110     
========================================
+ Hits        61798    61906   +108     
- Misses       1884     1886     +2     

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