feat(storage): implement transport, connector, worker

[vsharonlynn]

This PR comes after PR #5932.

[gemini-code-assist]

Code Review

This pull request introduces the appendable_object_writer module and supporting bidi_write infrastructure to enable bidirectional streaming writes for Google Cloud Storage. Key feedback includes avoiding .expect() on parameter conversions to prevent panics, simplifying complex string-folding logic, removing an unnecessary clone of the request object, and ensuring the background worker fails loudly with an explicit error if the stream closes unexpectedly while flushes are pending.

[codecov]

Codecov Report

❌ Patch coverage is 95.36862% with 49 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.73%. Comparing base (5e91002) to head (9cca986).

Files with missing lines	Patch %	Lines
src/storage/src/storage/bidi_write/worker.rs	82.03%	23 Missing ⚠️
src/storage/src/storage/bidi_write/transport.rs	96.03%	12 Missing ⚠️
src/storage/src/storage/bidi_write/connector.rs	98.03%	10 Missing ⚠️
src/storage/src/storage/bidi_write/redirect.rs	95.50%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5956      +/-   ##
==========================================
+ Coverage   97.72%   97.73%   +0.01%     
==========================================
  Files         242      246       +4     
  Lines       60882    61921    +1039     
==========================================
+ Hits        59495    60519    +1024     
- Misses       1387     1402      +15     

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[joshuatants]

We're gone through this extensively offline.