feat(storage): Add full object checksum validation for appendable uploads

[v-pratap]

No description provided.

[gemini-code-assist]

Code Review

This pull request adds support for CRC32C checksum validation during asynchronous uploads, specifically during finalization, resume, and flush operations. It updates the AsyncWriter interface and its underlying connection implementations to accept and verify expected checksums, and enhances HashFunction to support state retrieval and restoration. The review feedback highlights three critical issues: a compilation error in AsyncWriterConnectionImpl::Finalize caused by inconsistent return types in a lambda, and two data races in AsyncWriterConnectionResumed where impl_ is accessed concurrently without holding the appropriate mutex lock.

[codecov]

Codecov Report

❌ Patch coverage is 80.99415% with 65 lines in your changes missing coverage. Please review.
✅ Project coverage is 92.21%. Comparing base (f1a8e57) to head (1399a03).

Files with missing lines	Patch %	Lines
...torage/internal/async/writer_connection_resumed.cc	75.26%	23 Missing ⚠️
...le/cloud/storage/internal/async/connection_impl.cc	44.82%	16 Missing ⚠️
google/cloud/storage/internal/hash_function_impl.h	14.28%	12 Missing ⚠️
...orage/internal/async/writer_connection_buffered.cc	54.54%	5 Missing ⚠️
google/cloud/storage/internal/hash_function.h	0.00%	4 Missing ⚠️
google/cloud/storage/async/writer.cc	71.42%	2 Missing ⚠️
...torage/internal/async/writer_connection_tracing.cc	60.00%	2 Missing ⚠️
...e/internal/async/writer_connection_resumed_test.cc	98.87%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #16219      +/-   ##
==========================================
- Coverage   92.23%   92.21%   -0.03%     
==========================================
  Files        2265     2265              
  Lines      210205   210340     +135     
==========================================
+ Hits       193888   193967      +79     
- Misses      16317    16373      +56     

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