impl(v3): switch GCS to use GUAC

[scotthart]

This is second (and hopefully last PR to decommission the GCS specific OAuth2 API and use the GUAC API instead.

This PR:

• updates the tests previously using the GCS OAuth implementation to now use the GUAC implementation.
• updates the v3 migration guide
• deletes the GCS specific OAuth2 files
• updates the storage abi dump

[gemini-code-assist]

Summary of Changes

Hello @scotthart, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request completes the transition of the Google Cloud Storage client library's authentication mechanism from its legacy GCS-specific OAuth2 implementation to the unified Google Unified Authentication Client (GUAC) API. This change simplifies the authentication codebase, removes deprecated components, and aligns GCS with a more consistent and modern authentication strategy across Google Cloud client libraries. The migration guide and various tests have been updated to reflect these significant internal changes.

Highlights

• Migration to GUAC API: The Google Cloud Storage (GCS) client library has been updated to use the Google Unified Authentication Client (GUAC) API, replacing the GCS-specific OAuth2 API.
• Deprecated GCS OAuth2 Removal: All GCS-specific OAuth2 credential files and related internal components have been removed from the codebase, including oauth2/anonymous_credentials, oauth2/authorized_user_credentials, oauth2/compute_engine_credentials, oauth2/google_credentials, and oauth2/service_account_credentials.
• V3 Migration Guide Update: The v3 migration guide has been updated with new sections detailing the removal of Oauth2CredentialsOption and CreateServiceAccountCredentialsFromFilePath, providing clear 'Before' and 'After' code examples for migrating to the GUAC equivalents.
• Credential Handling Refinement: Internal credential handling logic has been centralized and streamlined, with the introduction of ApplyServiceAccountCredentialsInfoOverrides and a WrapRestCredentials class to adapt GUAC credentials for GCS client use.
• Test Suite Updates: Numerous unit and integration tests across the GCS client library have been updated to reflect the new GUAC-based authentication mechanisms, ensuring compatibility and correct behavior.

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[gemini-code-assist]

Code Review

This pull request is a significant and valuable refactoring that switches the GCS client to use the unified authentication client (GUAC), decommissioning the GCS-specific OAuth2 API. This aligns the GCS client with other C++ client libraries and simplifies the codebase by removing a lot of deprecated code. The changes correctly update client construction, authentication logic in tests and benchmarks, and the v3 migration guide. My review found a few typos in the code examples within the migration guide, for which I've provided suggestions. Overall, the changes are well-executed and improve the maintainability of the library.

[codecov]

Codecov Report

❌ Patch coverage is 87.96992% with 16 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.82%. Comparing base (d8fe957) to head (0991429).
⚠️ Report is 1 commits behind head on prepare-for-v3.0.0.

Files with missing lines	Patch %	Lines
...ts/service_account_credentials_integration_test.cc	0.00%	10 Missing ⚠️
.../cloud/storage/benchmarks/throughput_experiment.cc	0.00%	5 Missing ⚠️
...oud/internal/oauth2_service_account_credentials.cc	90.90%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15890      +/-   ##
======================================================
- Coverage               92.84%   92.82%   -0.02%     
======================================================
  Files                    2391     2360      -31     
  Lines                  218505   216610    -1895     
======================================================
- Hits                   202881   201079    -1802     
+ Misses                  15624    15531      -93     

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