impl(oauth2): add methods for returning multiple auth related http headers

[scotthart]

This PR adds methods for retrieving the authorization and x-allowed-locations headers individually, as well as, the AuthenticationHeaders method as a single place to get all the auth related headers for RPC calls.

[gemini-code-assist]

Code Review

This pull request refactors the credential authentication system to support multiple HTTP headers, replacing the single AuthenticationHeader with a vector-based AuthenticationHeaders method and adding specific logic for Authorization and AllowedLocations. A review comment suggests that errors from AllowedLocations should be handled more explicitly—either by logging or propagation—to avoid sending requests with missing headers that might cause ambiguous service failures.

[gemini-code-assist]

The current implementation silently ignores any errors returned by AllowedLocations. While the comment suggests this is intentional for credential types that don't support it, an actual Status error (other than OK) from an implementation that does attempt to provide it might indicate a transient failure (e.g., metadata server timeout). Swallowing such errors could lead to requests being sent without required headers, potentially causing confusing 403 or 400 errors from the service. Consider if specific error codes should be propagated or at least logged.

References

1. Prefer defensive code, such as explicit ok() checks, even if they seem redundant based on the current implementation of a framework, as the framework's contract may change in the future.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.69%. Comparing base (fdbec33) to head (e64f424).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16064      +/-   ##
==========================================
- Coverage   92.70%   92.69%   -0.01%     
==========================================
  Files        2343     2343              
  Lines      216644   216664      +20     
==========================================
+ Hits       200838   200846       +8     
- Misses      15806    15818      +12     

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