refactor(gax): extract make_credentials and add_auth_headers

[joshuatants]

For #5991

[gemini-code-assist]

Code Review

This pull request refactors the gRPC client by moving the make_credentials and add_auth_headers helper functions from grpc.rs to grpc_helpers.rs and adding corresponding unit tests. The feedback suggests replacing .unwrap() with .expect() in the newly added tests to comply with the repository's style guide regarding error handling in tests.

[codecov]

Codecov Report

❌ Patch coverage is 98.88889% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 97.71%. Comparing base (5a966c5) to head (3f5d199).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/gax-internal/src/grpc/grpc_helpers.rs	98.83%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6003   +/-   ##
=======================================
  Coverage   97.71%   97.71%           
=======================================
  Files         244      244           
  Lines       61074    61139   +65     
=======================================
+ Hits        59680    59744   +64     
- Misses       1394     1395    +1     

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[coryan]

No action needed: I do not like the grpc_ prefix in this file, it is redundant.