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 91.96787% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.56%. Comparing base (0de1339) to head (6e5412a).
⚠️ Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
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	95.74%	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   97.59%   97.56%   -0.03%     
==========================================
  Files         222      222              
  Lines       46609    46826     +217     
==========================================
+ Hits        45486    45685     +199     
- Misses       1123     1141      +18     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[coryan]

Is it time to close this PR? It is several weeks old by now.

[alvarowolfx]

Is it time to close this PR? It is several weeks old by now.

sure, I'll reopen later when time comes to make the API public for overriding universe domain