GCS client library migration in Java SDK - part 3

[shunping]

• Add open and create methods for GcsUtilV2.
• Add bridge channel classes to ensure we keep the function signature as much as we can.

[gemini-code-assist]

Summary of Changes

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This pull request, part three of the GCS client library migration, focuses on enabling fundamental read and write capabilities using the updated GcsUtilV2. It introduces new methods for opening and creating GCS files, along with adapter classes to ensure seamless integration with existing Java I/O channel interfaces. This change is crucial for progressing the migration while maintaining functional parity and allowing for a controlled rollout of the new client library features.

Highlights

• GcsUtilV2 Read/Write Operations: Implemented open and create methods within GcsUtilV2 to enable direct read and write operations to Google Cloud Storage using the new client library.
• Channel Bridge Classes: Introduced GcsSeekableByteChannel and GcsWritableByteChannel to act as bridge classes, adapting the com.google.cloud library's ReadChannel and WriteChannel to the standard Java SeekableByteChannel and WritableByteChannel interfaces, maintaining API compatibility.
• GcsUtil Delegation: Modified the existing GcsUtil class to conditionally delegate open and create calls to the new GcsUtilV2 implementation when the V2 delegate is initialized, facilitating a gradual migration.
• Integration Tests: Added new integration tests (testRead and testWriteAndRead) in GcsUtilParameterizedIT to verify the functionality of the new GCS read and write operations.

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