feat(storage): support DirectPath over Interconnect in GCS gRPC

[kalragauri]

This PR adds support for DirectPath over Interconnect in the Cloud Storage gRPC client library.

Summary of Changes

• Introduced storage_experimental::DirectPathXdsOverInterconnectOptionand support for the GOOGLE_CLOUD_ENABLE_DIRECT_PATH_XDS_OVER_INTERCONNECT environment variable.
• Configured storage_internal::DefaultOptionsGrpc to resolve target endpoint google-c2p:///storage-direct.googleapis.com?force-xds with authority storage.googleapis.com when enabled, bypassing GCE environment checks.

[gemini-code-assist]

Code Review

This pull request introduces the DirectPathXdsOverInterconnectOption and the corresponding environment variable GOOGLE_CLOUD_ENABLE_DIRECT_PATH_XDS_OVER_INTERCONNECT to allow configuring DirectPath over Interconnect. When enabled, the client targets google-c2p:///storage-direct.googleapis.com?force-xds with standard TLS. Comprehensive unit tests have been added to cover various scenarios and overrides. Feedback is provided regarding a style guide violation where auto is used for a boolean type instead of an explicit bool const declaration.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.26%. Comparing base (6c82dc0) to head (90b473f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16408      +/-   ##
==========================================
+ Coverage   92.25%   92.26%   +0.01%     
==========================================
  Files        2246     2246              
  Lines      212291   212416     +125     
==========================================
+ Hits       195841   195995     +154     
+ Misses      16450    16421      -29     

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