Deprecating openai assistant agent. Apply version conditioned import for open ai version < 1.83

[Eric Zhu (ekzhu)]

This PR deprecates OpenAIAssistantAgent. The implications:

1. If the Python environment has openai<1.83, the class will still be available and usable as before from autogen_ext.agents.openai.OpenAIAssistantAgent, however, it will raise a deprecation warning asking user to migrate to OpenAIAgent.
2. If the Python environment has openai>=1.83, then the class will be unavaible for import from the autogen_ext.agents.openai module.
3. For development environment, the module will not be type checked regardless of openai client library version.

See the release note from the openai client library: https://github.com/openai/openai-python/releases/tag/v1.83.0

[codecov]

Codecov Report

Attention: Patch coverage is 78.78788% with 7 lines in your changes missing coverage. Please review.

Project coverage is 79.87%. Comparing base (413d8f1) to head (39cb5dc).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...ogen-ext/src/autogen_ext/agents/openai/__init__.py	73.68%	5 Missing ⚠️
...togen_ext/agents/openai/_openai_assistant_agent.py	85.71%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6827      +/-   ##
==========================================
- Coverage   80.81%   79.87%   -0.95%     
==========================================
  Files         233      233              
  Lines       18074    18105      +31     
==========================================
- Hits        14607    14461     -146     
- Misses       3467     3644     +177     

Flag	Coverage Δ
unittests	79.87% <78.78%> (-0.95%)	⬇️

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