feat(storage): introduce basic appendable upload benchmark scenario

[vsharonlynn]

Issue #5716.

[gemini-code-assist]

Code Review

This pull request introduces a new benchmark tool, storage-benchmark-appendable-object, to measure Cloud Storage appendable upload performance, and implements tracing annotations for the AppendableObjectWriter along with corresponding unit tests. The reviewer feedback highlights several key improvement opportunities: optimizing memory usage in the benchmark scenario by reusing chunks instead of pre-allocating them all in a Vec to prevent potential OOM crashes; allowing fewer than 100 measured iterations to facilitate quicker local testing; fixing shell argument forwarding in run_all.sh using bash arrays to prevent word-splitting bugs; and optimizing the flush tracing implementation by recording the persisted size directly from the returned result.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.57%. Comparing base (b3a930b) to head (81039ea).
⚠️ Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6149      +/-   ##
==========================================
- Coverage   96.57%   96.57%   -0.01%     
==========================================
  Files         264      264              
  Lines       66577    66577              
==========================================
- Hits        64297    64296       -1     
- Misses       2280     2281       +1     

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