impl: enable GDCH Service Account credentials for gRPC endpoints

[scotthart]

This PR adds support for creating GDCH Service Account Credentials if gRPC v1.84.0 or later is being used. Manual testing with v1.84.0 has been done.

As part of the v3 major version, nlohmann::json became a required dependency. Adding it to the grpc library is in line with that change.

[gemini-code-assist]

Code Review

This pull request implements support for GDCH Service Account Credentials in gRPC, which requires gRPC version 1.84.0 or greater. It introduces dependencies on nlohmann_json, updates Bazel and CMake build configurations, implements the credential visitor logic, and adds comprehensive unit and integration tests. The review feedback correctly identifies three important areas for improvement: avoiding potential C++ exceptions when parsing JSON to comply with library safety guidelines, failing loudly instead of silently swallowing errors when a specified CA certificate file cannot be opened, and removing default parameters in an internal namespace within the test file to adhere to the repository style guide.

[codecov]

Codecov Report

❌ Patch coverage is 52.17391% with 11 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.28%. Comparing base (6e0a08e) to head (0ed4943).

Files with missing lines	Patch %	Lines
...le/cloud/internal/unified_grpc_credentials_test.cc	60.00%	8 Missing ⚠️
google/cloud/internal/unified_grpc_credentials.cc	0.00%	3 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main   #16428   +/-   ##
=======================================
  Coverage   92.28%   92.28%           
=======================================
  Files        2246     2246           
  Lines      212894   212913   +19     
=======================================
+ Hits       196474   196493   +19     
  Misses      16420    16420           

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