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

❌ Patch coverage is 13.23529% with 59 lines in your changes missing coverage. Please review.
✅ Project coverage is 96.35%. Comparing base (30c6e15) to head (4301a07).

Files with missing lines	Patch %	Lines
src/storage/src/storage/transport.rs	0.00%	45 Missing ⚠️
src/storage/src/storage/stub.rs	0.00%	10 Missing ⚠️
src/storage/src/storage/open_appendable_object.rs	69.23%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6491      +/-   ##
==========================================
- Coverage   96.41%   96.35%   -0.07%     
==========================================
  Files         303      303              
  Lines       85422    85490      +68     
==========================================
+ Hits        82362    82371       +9     
- Misses       3060     3119      +59     

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