impl(storage): decommission GCS specific OAuth2 API

[scotthart]

This PR focuses on removing the deprecated GCS OAuth wrappers around the OAuth implementation in the common library. In addition to deleting a lot of code, it also adds a few features to the common OAuth and Credentials libraries to maintain feature parity.

Highlights include:

• replacing uses of Oauth2CredentialsOption with UnifiedCredentialsOption and removes Oauth2CredentialsOption
• adding support for creating ServiceAccountCredentials from either JSON or P12 files to common Credentials
• ensuring that GCS's special behavior regarding self signed JWTs and additional scopes is preserved via oauth2_internal::DisableSelfSignedJWTOption
• maintains GCS feature parity regarding signing of blobs, urls, documents, etc.

[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 significantly refactors the authentication infrastructure within the Google Cloud C++ client library for Google Cloud Storage. The primary goal is to consolidate authentication logic by removing GCS-specific OAuth2 implementations and fully adopting the unified google::cloud::Credentials interface. This change simplifies the codebase, enhances maintainability, and provides a more consistent developer experience across Google Cloud services. Additionally, it introduces a convenient new method for initializing service account credentials directly from file paths.

Highlights

• Decommissioned GCS-specific OAuth2 API: The entire google::cloud::storage::oauth2 namespace and its associated credential implementations have been removed, streamlining the authentication layer.
• Transition to Unified Credentials: The Google Cloud Storage (GCS) client library now exclusively utilizes the unified google::cloud::Credentials API for all authentication mechanisms, reducing code duplication and improving consistency.
• New Service Account Credentials from File API: A new public API, MakeServiceAccountCredentialsFromFile, has been introduced in the unified credentials library, enabling the creation of service account credentials directly from a file path.
• Enhanced Internal Credential Handling: Internal gRPC and REST credential visitors have been updated to seamlessly process service account configurations provided via file paths, ensuring consistent behavior across different transports.
• Removed Oauth2CredentialsOption: The GCS-specific Oauth2CredentialsOption has been removed from the client options, with UnifiedCredentialsOption now serving as the standard for credential configuration.

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[gemini-code-assist]

Code Review

This pull request decommissions the GCS-specific OAuth2 API, replacing it with a unified credentials implementation. This is a significant and positive refactoring that centralizes authentication logic. The changes are extensive, involving the removal of many old credential-related files and updating call sites to use the new APIs. My review found a couple of places where error handling for file operations could be improved to provide better diagnostics. Otherwise, the changes look correct and consistent with the goal of the PR.

[codecov]

Codecov Report

❌ Patch coverage is 77.47253% with 41 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.82%. Comparing base (c201e56) to head (1345e57).

Files with missing lines	Patch %	Lines
...oud/internal/oauth2_service_account_credentials.cc	65.90%	15 Missing ⚠️
...ts/service_account_credentials_integration_test.cc	0.00%	10 Missing ⚠️
google/cloud/internal/unified_grpc_credentials.cc	56.25%	7 Missing ⚠️
.../cloud/storage/benchmarks/throughput_experiment.cc	0.00%	5 Missing ⚠️
google/cloud/internal/unified_rest_credentials.cc	75.00%	4 Missing ⚠️

Additional details and impacted files

@@                  Coverage Diff                   @@
##           prepare-for-v3.0.0   #15883      +/-   ##
======================================================
- Coverage               92.87%   92.82%   -0.05%     
======================================================
  Files                    2391     2360      -31     
  Lines                  218434   216582    -1852     
======================================================
- Hits                   202866   201045    -1821     
+ Misses                  15568    15537      -31     

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