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 92.29%. Comparing base (fdea5ac) to head (c7c7c6f).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #16408    +/-   ##
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  Coverage   92.28%   92.29%            
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  Files        2246     2246            
  Lines      212913   213095   +182     
========================================
+ Hits       196491   196677   +186     
+ Misses      16422    16418     -4     

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