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 80.21201% with 56 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.91%. Comparing base (aaae947) to head (8fe201f).
⚠️ Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
src/auth/src/credentials/external_account.rs	37.50%	15 Missing ⚠️
src/auth/src/universe_domain.rs	46.15%	7 Missing ⚠️
src/auth/src/credentials/impersonated.rs	53.84%	6 Missing ⚠️
src/auth/src/credentials.rs	80.76%	5 Missing ⚠️
src/auth/src/mds/client.rs	66.66%	5 Missing ⚠️
src/auth/src/credentials/mds.rs	80.00%	4 Missing ⚠️
src/auth/src/credentials/service_account.rs	73.33%	4 Missing ⚠️
src/gax/src/client_builder.rs	20.00%	4 Missing ⚠️
src/auth/src/credentials/user_account.rs	0.00%	3 Missing ⚠️
src/gax-internal/src/host.rs	97.50%	1 Missing ⚠️
... and 2 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5146      +/-   ##
==========================================
- Coverage   98.02%   97.91%   -0.11%     
==========================================
  Files         214      216       +2     
  Lines       44587    44839     +252     
==========================================
+ Hits        43705    43905     +200     
- Misses        882      934      +52     

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