feat(storage): add code samples

[vsharonlynn]

Issue #5716.

[gemini-code-assist]

Code Review

This pull request introduces new example files demonstrating appendable object operations (finalize, pause/resume, read tail, and write) for Google Cloud Storage, gated under the unstable-stream feature. The feedback suggests improving code readability in lib.rs by using imported types directly instead of fully qualified paths, and removing redundant flush() calls before close() and finalize() operations since those methods perform flushing internally.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.03%. Comparing base (d7c735b) to head (318502d).
⚠️ Report is 8 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6225      +/-   ##
==========================================
- Coverage   96.03%   96.03%   -0.01%     
==========================================
  Files         269      269              
  Lines       67566    67566              
==========================================
- Hits        64888    64884       -4     
- Misses       2678     2682       +4     

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