impl(v3): add support for creating OAuth SA creds from files

[scotthart]

This is the first of a series of PRs (to avoid 1 mega-PR), to decommission the GCS specific OAuth2 API and use the GUAC API instead.

This PR addresses feature parity by porting support for reading service account credentials from JSON and PKCS12 files from the GCS OAuth implementation to the GUAC implementation.

A subsequent PR will rewire the existing GCS OAuth tests for this functionality to use this GUAC implementation.

[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 is a foundational step in transitioning the OAuth2 API from a GCS-specific implementation to the more versatile GUAC API. It introduces the essential capability to load service account credentials from external JSON and PKCS#12 files, ensuring feature parity with the previous system. The changes involve significant internal refactoring to support these file-based configurations consistently across both gRPC and REST transport layers, paving the way for the eventual decommissioning of the older API.

Highlights

• File-based Service Account Credentials: Added a new public function MakeServiceAccountCredentialsFromFile to create OAuth service account credentials directly from a file path, supporting both JSON and PKCS#12 formats.
• Refactored ServiceAccountConfig: The internal ServiceAccountConfig class was updated to use absl::optional<std::string> for both JSON content and file path, allowing it to store credentials sourced from either a string or a file.
• Internal Credential Creation Helpers: Introduced a suite of internal helper functions (CreateServiceAccountCredentialsFromJsonContents, FromJsonFilePath, FromP12FilePath, FromFilePath) to centralize the logic for parsing and creating service account credentials from various sources.
• Unified Credential Handling: The gRPC and REST credential mapping logic (unified_grpc_credentials.cc and unified_rest_credentials.cc) was updated to correctly interpret and utilize the new file-based service account configurations.
• Disable Self-Signed JWT Option: Added a DisableSelfSignedJWTOption to provide more granular control over when self-signed JSON Web Tokens (JWTs) are used, particularly for services with stricter requirements like Cloud Storage.

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[gemini-code-assist]

Code Review

This pull request is a solid step towards unifying credential handling by adding support for creating service account credentials from files. The changes are well-structured, introducing a new factory function MakeServiceAccountCredentialsFromFile and updating internal configurations to handle either a JSON object string or a file path. The implementation correctly dispatches to file-based loading for both gRPC and REST transports, respecting transport-specific limitations.

I've identified a potential crash in a test utility due to unsafe optional dereferencing, which should be addressed. I've also pointed out a minor code duplication that could be refactored for better maintainability. Overall, this is a good contribution.

[codecov]

Codecov Report

❌ Patch coverage is 43.15789% with 54 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.85%. Comparing base (c201e56) to head (e02f3ec).
⚠️ Report is 1 commits behind head on prepare-for-v3.0.0.

Files with missing lines	Patch %	Lines
...oud/internal/oauth2_service_account_credentials.cc	37.50%	30 Missing ⚠️
google/cloud/internal/unified_grpc_credentials.cc	25.00%	12 Missing ⚠️
google/cloud/internal/unified_rest_credentials.cc	58.33%	5 Missing ⚠️
google/cloud/credentials.cc	25.00%	3 Missing ⚠️
...cloud/storage/internal/unified_rest_credentials.cc	40.00%	3 Missing ⚠️
google/cloud/testing_util/credentials.h	66.66%	1 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15887      +/-   ##
======================================================
- Coverage               92.87%   92.85%   -0.03%     
======================================================
  Files                    2391     2391              
  Lines                  218434   218505      +71     
======================================================
+ Hits                   202866   202882      +16     
- Misses                  15568    15623      +55     

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