feat(gax): allow Google API client header to be overrided

[alvarowolfx]

This will allow veneers to change header from GAPIC to veneer (gccl).

Towards #2138

[alvarowolfx]

need to be done in sync with googleapis/librarian#7514 removed template changes

[gemini-code-assist]

Code Review

This pull request refactors the resolution and handling of the x-goog-api-client header across multiple generated transport modules. It introduces helper functions to resolve these headers for both gRPC and REST requests, improves code cleanliness by removing redundant #[allow(unused_imports)] attributes, and updates the ReqwestClient to support these changes. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.67%. Comparing base (bec0ff2) to head (3315904).
⚠️ Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6665      +/-   ##
==========================================
- Coverage   96.67%   96.67%   -0.01%     
==========================================
  Files         308      308              
  Lines       97938    97988      +50     
==========================================
+ Hits        94683    94728      +45     
- Misses       3255     3260       +5     

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[alvarowolfx]

/gcbrun