docs(storage): add sample for open appendable object send_and_append

[vsharonlynn]

Issue #5716 .

This PR follows PR #6490 .

[gemini-code-assist]

Code Review

This pull request introduces the send_and_append feature for the open_appendable_object API in the Google Cloud Storage Rust SDK, allowing clients to send an initial chunk of data directly within the opening stream request. The changes update the AppendObjectSpecState enum to track the initial payload, add a new connect_open_and_append method in the connector, and implement support in AppendableObjectWriterTransport to handle and potentially split the initial chunk. A new example and corresponding unit tests have also been added to verify the functionality. I have no feedback to provide as there are no review comments.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.52%. Comparing base (7684ebe) to head (d438320).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6491   +/-   ##
=======================================
  Coverage   96.52%   96.52%           
=======================================
  Files         304      304           
  Lines       92054    92054           
=======================================
  Hits        88858    88858           
  Misses       3196     3196           

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[vsharonlynn]

gcb rust-sdk-testing is crashing with:

thread 'tests::object_examples' (85415) has overflowed its stack
fatal runtime error: stack overflow, aborting

I sent out a PR to prevent this storage-sample stack overflow: #6660 .
Will revisit this PR after #6660 is merged.