feat(storage): implement appendable upload background worker

[vsharonlynn]

This PR comes after #5956.

[gemini-code-assist]

Code Review

This pull request introduces the internal infrastructure for Appendable Object Write (Bidi Write) in Google Cloud Storage, including a Connector to handle the initial handshake and redirects, and a background Worker to manage the live gRPC stream and upload intents. The feedback highlights two key issues: first, preconditions (if_metageneration_match and if_metageneration_not_match) are lost when transitioning from WriteObjectSpec to AppendObjectSpec in the connector; second, the worker exits silently with Ok(()) if the server closes the stream unexpectedly while the client is idle, which should instead be treated as an error.

[codecov]

Codecov Report

❌ Patch coverage is 95.06579% with 15 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.05%. Comparing base (53d123d) to head (5068b51).

Files with missing lines	Patch %	Lines
src/storage/src/storage/bidi_write/worker.rs	95.06%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6001      +/-   ##
==========================================
+ Coverage   97.04%   97.05%   +0.01%     
==========================================
  Files         253      254       +1     
  Lines       63682    63986     +304     
==========================================
+ Hits        61798    62103     +305     
+ Misses       1884     1883       -1     

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[vsharonlynn]

I'll merge this first to meet deadlines. Please feel free to leave feedback. I will incorporate them in a separate PR. Thank you.