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 96.98%. Comparing base (b368afc) to head (b76a64f).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #6491   +/-   ##
=======================================
  Coverage   96.97%   96.98%           
=======================================
  Files         325      325           
  Lines      106402   106402           
=======================================
+ Hits       103188   103194    +6     
+ Misses       3214     3208    -6     

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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.