feat(storage): add support for DirectPath over Interconnect

[nidhiii-27]

• DirectPath over Interconnect Support (GAX): Added the attemptDirectPathXdsOverInterconnect option to enable on-premise xDS name resolution via the google-c2p:///<service>?force-xds target scheme.
• GCP Platform Check Bypass: Modified DirectPath validation logic to bypass standard Google Compute Engine (GCE) environment checks when Interconnect is enabled.
• Custom URI Validation: Enhanced endpoint validation to support and validate custom target URI formats containing the :/// syntax (e.g. google-c2p:///).
• Storage Layer Integration: Added configuration options to GrpcStorageOptions to automatically rewrite hosts to storage-direct.googleapis.com and override the request authority to storage.googleapis.com for secure TLS handshakes.

[gemini-code-assist]

Code Review

This pull request introduces support for DirectPath xDS over Interconnect (on-premise xDS name resolution) in GrpcStorageOptions and InstantiatingGrpcChannelProvider, allowing GCE environment checks to be bypassed when enabled. It also updates endpoint validation to support custom URI schemes like google-c2p:/// and adds corresponding tests. The reviewer suggests dynamically adjusting the log level for the DirectPath fallback warning to avoid excessive warning spam in non-GCE environments, recommending Level.WARNING on GCE and Level.FINE elsewhere.

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
24.1% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud

[sonarqubecloud]

Quality Gate failed for 'gapic-generator-java-root'

Failed conditions
0.0% Coverage on New Code (required ≥ 80%)

See analysis details on SonarQube Cloud