feat(auth): make GDCH authentication pub

[alvarowolfx]

Towards #5584

[gemini-code-assist]

Code Review

This pull request promotes the Google Distributed Cloud (GDC) authentication feature from internal to public by renaming the "__gdch" feature to "gdch" and exposing the "gdch" module and its "Builder". The changes include extensive documentation, usage examples, and explicit error handling that directs users to the "gdch::Builder" when GDC service accounts are detected in general credential flows. Integration tests have been added to verify these new public APIs and error paths. I have no feedback to provide.

[codecov]

Codecov Report

❌ Patch coverage is 25.00000% with 6 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.76%. Comparing base (ac4115f) to head (bea3f66).

Files with missing lines	Patch %	Lines
src/auth/src/credentials.rs	16.66%	5 Missing ⚠️
src/auth/src/credentials/idtoken.rs	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5614      +/-   ##
==========================================
- Coverage   97.77%   97.76%   -0.01%     
==========================================
  Files         221      221              
  Lines       51107    51115       +8     
==========================================
+ Hits        49969    49972       +3     
- Misses       1138     1143       +5     

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[alvarowolfx]

cc @howardjohn this PR makes the gdch authentication available/public. It will be under the gdch feature.

[howardjohn]

I tested this on my environment and it works as expected. thanks!