Support openai assistant v2 API

[IANTHEREAL]

Why are these changes needed?

This PR introduces support for the newly released OpenAI Assistant API v2, incorporating significant changes outlined in the migration guide. We aim to ensure compatibility with the existing API v1 without disrupting current implementations. The key modifications include:

1. Dual API Support: Refactored the assistant creation and update functions to support both API v1 and v2 seamlessly.
2. Enhanced File Handling: When a user calls the GPT assistant agent with file_ids, the code now attempts to create a vector store and utilizes file_search. This enhancement maintains compatibility with the existing user codebase.
3. Updated Testing Suite: Expanded the test suite to verify functionality against the API v2 specifications while preserving a subset of tests for API v1 to ensure ongoing compatibility.

By making these changes, it aim to leverage the advancements in API v2 while maintaining a smooth transition for existing applications still utilizing API v1.

Related issue number

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://microsoft.github.io/autogen/docs/Contribute#documentation to build and test documentation locally.
• I've added tests (if relevant) corresponding to the changes introduced in this PR.
• I've made sure all auto checks have passed.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 10.14493% with 62 lines in your changes are missing coverage. Please review.

Project coverage is 33.20%. Comparing base (ded2d61) to head (d680d4f).
Report is 1 commits behind head on main.

Files	Patch %	Lines
autogen/oai/openai_utils.py	11.86%	52 Missing ⚠️
autogen/agentchat/contrib/gpt_assistant_agent.py	0.00%	10 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2466      +/-   ##
==========================================
- Coverage   33.33%   33.20%   -0.14%     
==========================================
  Files          83       83              
  Lines        8636     8692      +56     
  Branches     1835     1852      +17     
==========================================
+ Hits         2879     2886       +7     
- Misses       5516     5565      +49     
  Partials      241      241              

Flag	Coverage Δ
unittests	33.20% <10.14%> (-0.14%)	⬇️

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[ekzhu]

Are the new changes compatible with older version of assistant API?

[IANTHEREAL]

Are the new changes compatible with older version of assistant API?

Yes, it has been done with the maximum backward compatibility. If the user uses the old file_ids to create the assistant, the assistant can still be created successfully. However, if the old and new configurations are mixed, an error will be reported.