feat(storage): add builder and writer skeleton for open appendable object

[vsharonlynn]

This follows PR #5697.

[gemini-code-assist]

Code Review

This pull request introduces a skeleton implementation for appendable object uploads, including the OpenAppendableObject request builder and the AppendableObjectWriter utility. Feedback focuses on the structural requirement for the writer to hold a reference to the storage transport layer (the stub) to facilitate bidirectional streaming. Additionally, the reviewer noted that the use of #[allow(dead_code)] without a corresponding TODO or rationale violates the repository's style guide regarding temporary lint suppressions.

[codecov]

Codecov Report

❌ Patch coverage is 68.33333% with 95 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.75%. Comparing base (6e7650c) to head (b1eba40).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/storage/src/storage/append_object/builder.rs	73.47%	74 Missing ⚠️
src/storage/src/storage/append_object/writer.rs	0.00%	21 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #5699      +/-   ##
==========================================
- Coverage   97.91%   97.75%   -0.16%     
==========================================
  Files         226      228       +2     
  Lines       55347    55647     +300     
==========================================
+ Hits        54191    54398     +207     
- Misses       1156     1249      +93     

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