feat(storage): Add full object checksum validation at finalization for json path

[v-pratap]

This PR modifies the client library to send the x-goog-hash header, containing crc32c= and/or md5= values, if available, when sending the final chunk of a resumable upload.

Implementation Details
The core of the change involves coordinating hash calculation between the ObjectWriteStreambuf and the RestStub. The hash_function within ObjectWriteStreambuf maintains the running checksum state for the entire upload.

RestStub::UploadChunk:
For intermediate chunks (when last_chunk() is false), UploadChunk receives a valid hash_function_ptr from ObjectWriteStreambuf. It calls hash_function->Update() to incorporate the chunk's payload into the running checksum.
For the final chunk (when last_chunk() is true), UploadChunk receives a nullptr for hash_function_ptr but receives the final, computed object hashes via request.known_object_hashes(). It then adds the x-goog-hash header(s) to the HTTP request using these final hashes.

ObjectWriteStreambuf::FlushFinal:
This method is called when Close() is invoked on the stream.
It first calls hash_function->Update() with any data still pending in its internal buffer.

[gemini-code-assist]

Summary of Changes

Hello @v-pratap, 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 enhances the Google Cloud Storage client library by implementing full object checksum validation for resumable uploads. It introduces mechanisms to calculate and transmit crc32c and md5 checksums in the x-goog-hash HTTP header during the finalization step of an upload, thereby bolstering data integrity and reliability for large object transfers.

Highlights

• Checksum Header Integration: The client library now sends x-goog-hash headers containing crc32c and/or md5 values with the final chunk of resumable uploads, enhancing data integrity.
• Hash Calculation Coordination: Hash calculation logic is coordinated between ObjectWriteStreambuf and RestStub, ensuring that the final, computed object hashes are correctly passed and included in the HTTP request.
• Enhanced Test Coverage: Comprehensive unit tests have been added to verify the correct behavior of hash computation, passing, and header inclusion for various scenarios, including intermediate and final chunks, and different combinations of crc32c and md5.

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[gemini-code-assist]

Code Review

This pull request adds support for sending object checksums (CRC32C and MD5) in the final chunk of a resumable upload for the JSON/REST transport. The changes look good and are well-tested. I've found a couple of minor issues related to leftover debugging code that should be addressed before merging.

[codecov]

Codecov Report

❌ Patch coverage is 89.82036% with 17 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.95%. Comparing base (46f7e8b) to head (e233715).

Files with missing lines	Patch %	Lines
google/cloud/storage/internal/rest/stub_test.cc	80.88%	13 Missing ⚠️
google/cloud/storage/internal/object_requests.cc	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #15815      +/-   ##
==========================================
+ Coverage   92.94%   92.95%   +0.01%     
==========================================
  Files        2458     2458              
  Lines      227585   227725     +140     
==========================================
+ Hits       211531   211685     +154     
+ Misses      16054    16040      -14     

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