feat(storage): Expose Flush() in AsyncWriter

[shubham-up-47]

This change introduces a Flush() method to the AsyncWriter class, providing users with more control over the asynchronous upload process. If multiple Flush() calls are made, they are queued and processed sequentially. Each call returns a future that will be satisfied only when that specific flush operation (and all preceding ones) have been acknowledged by the service.

For unbuffered (StartUnbufferedUpload) and appendable (StartAppendableObjectUpload) uploads, this is implemented using a promise queue to serialize flush operations, ensuring thread safety for concurrent use. For buffered uploads (StartBufferedUpload), the Flush() method currently forwards to Write(). A more complete implementation that forces the buffer to be sent and handles concurrent calls correctly is planned for a follow-up PR.

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This change is 

[shubham-up-47]

/gcbrun

[codecov]

Codecov Report

❌ Patch coverage is 98.50746% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 93.05%. Comparing base (3962f01) to head (fea5648).
⚠️ Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
...oud/storage/tests/async_client_integration_test.cc	93.33%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##             main   #15555    +/-   ##
========================================
  Coverage   93.04%   93.05%            
========================================
  Files        2408     2408            
  Lines      220145   220278   +133     
========================================
+ Hits       204829   204970   +141     
+ Misses      15316    15308     -8     

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[bajajneha27]

Can we also add an integration test for explicit flush?

[shubham-up-47]

Can we also add an integration test for explicit flush?

Done.

[scotthart]

@scotthart reviewed 1 of 3 files at r1, 3 of 3 files at r4, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @bajajneha27)