impl(oauth2): add RegionalAccessBoundary Credentials decorator

[scotthart]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces the RegionalAccessBoundaryTokenManager to manage and cache routing tokens for Regional Access Boundaries (RAB) across several credential types, including Service Account and External Account credentials. The Credentials interface is refactored to support AllowedLocationsRequest, and the decoration logic is updated to integrate the new manager, controlled by a new testing macro. Feedback highlights the need to maintain C++14 compatibility by replacing C++17 features—such as std::variant, std::scoped_lock, and std::holds_alternative—with absl or standard C++14 alternatives. Furthermore, the retry logic should be expanded to include kDeadlineExceeded, and the AllowedLocations endpoint should be accessed using GET requests.

[codecov]

Codecov Report

❌ Patch coverage is 98.24561% with 7 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.70%. Comparing base (f46998c) to head (beeac5a).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...th2_regional_access_boundary_token_manager_test.cc	97.59%	5 Missing ⚠️
...l/oauth2_regional_access_boundary_token_manager.cc	97.82%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16081      +/-   ##
==========================================
+ Coverage   92.66%   92.70%   +0.03%     
==========================================
  Files        2348     2351       +3     
  Lines      217564   217954     +390     
==========================================
+ Hits       201612   202052     +440     
+ Misses      15952    15902      -50     

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