feat: Enable full object checksum validation for multi-chunk resumable uploads

[mahendra-google]

This PR enables full object checksum validation (specifically CRC32C) for JSON-based resumable uploads, ensuring end-to-end data integrity for multi-chunk transfers. Please see b/461996128

[gemini-code-assist]

Summary of Changes

Hello @mahendra-google, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request significantly enhances the reliability of resumable uploads by introducing comprehensive CRC32C checksum validation. It ensures that data integrity is maintained throughout the entire multi-chunk upload process, from initial session setup to the final data transfer, by verifying checksums both at the start and end of the upload.

Highlights

• CRC32C Validation: Implements full object CRC32C checksum validation for multi-chunk resumable uploads to ensure end-to-end data integrity.
• Session Initiation Check: Adds logic to validate provided CRC32C checksums during the session initiation phase, preventing uploads with mismatched checksums.
• Final Chunk Hashing: Automatically calculates and includes the CRC32C hash in the "X-Goog-Hash" header for the final chunk of an upload, if not already sent during initiation.
• Platform-Specific Hashing: Integrates System.IO.Hashing.Crc32 for efficient CRC32C calculation on .NET 6.0 and above, with a fallback for older frameworks.
• Comprehensive Testing: Introduces new test cases to cover various scenarios of CRC32C validation, including successful uploads and initiation failures due to checksum mismatches.

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[gemini-code-assist]

Code Review

This pull request adds support for CRC32C checksum validation for multi-chunk resumable uploads, which is a great feature for ensuring data integrity. The implementation looks solid, with good handling of different scenarios like seekable/non-seekable streams and framework-specific code paths using conditional compilation. The use of ArrayPool for buffering in the checksum calculation is a nice performance consideration.

I've added a few suggestions to improve the new tests, mainly to enhance their clarity and expand their coverage to include non-seekable streams. Overall, this is a well-executed feature addition.

[amanda-tarafa]

I've left a comment on the internal issue.