feat(auth): universe domain support - WIP

[alvarowolfx]

Towards #3646

This PR will be broken down into smaller ones. It's just experimentation and to show the big picture.

[codecov]

Codecov Report

❌ Patch coverage is 90.03831% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.75%. Comparing base (b43a714) to head (57eb168).

Files with missing lines	Patch %	Lines
src/auth/src/credentials/impersonated.rs	53.84%	6 Missing ⚠️
src/auth/src/mds/client.rs	93.61%	6 Missing ⚠️
src/auth/src/credentials.rs	80.76%	5 Missing ⚠️
src/auth/src/credentials/mds.rs	82.60%	4 Missing ⚠️
src/auth/src/credentials/user_account.rs	0.00%	3 Missing ⚠️
src/gax-internal/src/host.rs	97.50%	1 Missing ⚠️
src/gax-internal/src/http.rs	96.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5146      +/-   ##
==========================================
- Coverage   97.79%   97.75%   -0.04%     
==========================================
  Files         220      220              
  Lines       45938    46164     +226     
==========================================
+ Hits        44924    45127     +203     
- Misses       1014     1037      +23     

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