feat(storage): implement API, request builders, integration tests

[vsharonlynn]

This PR comes after PR #5956.

[gemini-code-assist]

Code Review

This pull request implements bidirectional streaming writes (Bidi Write) for Google Cloud Storage, introducing the AppendableObjectWriter along with request builders OpenAppendableObject and ReopenAppendableObject to support appending data to objects. It includes a background Worker to manage the gRPC stream, a Connector to handle connection handshakes and routing redirects, and comprehensive unit and integration tests. The review feedback highlights two opportunities in connector.rs to improve error handling by replacing .expect() calls with safe error propagation using ? when converting parameters to proto format.

[codecov]

Codecov Report

❌ Patch coverage is 85.81699% with 217 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.48%. Comparing base (5e91002) to head (5e066c1).

Files with missing lines	Patch %	Lines
src/storage/src/storage/open_appendable_object.rs	45.04%	111 Missing ⚠️
...rc/storage/src/storage/reopen_appendable_object.rs	68.64%	37 Missing ⚠️
src/storage/src/storage/bidi_write/worker.rs	82.03%	23 Missing ⚠️
src/storage/src/storage/stub.rs	0.00%	18 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 ⚠️
src/storage/src/storage/transport.rs	97.14%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5957      +/-   ##
==========================================
- Coverage   97.72%   97.48%   -0.24%     
==========================================
  Files         242      250       +8     
  Lines       60882    62351    +1469     
==========================================
+ Hits        59495    60782    +1287     
- Misses       1387     1569     +182     

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[vsharonlynn]

I ran

$ cargo +nightly public-api -p google-cloud-storage diff origin/main..HEAD

The output is:

Removed items from the public API
=================================
(none)

Changed items in the public API
===============================
(none)

Added items to the public API
=============================
(none)

[coryan]

This is too large for a single PR, please split.