support llm_config in AgentOptimizer

[skzhang1]

Why are these changes needed?

AgentOptimizer with OAI_CONFIG_LIST in its constructor is forcing users to learn the LLM config utilities.
Taking llm_config as inout is more convenient for users.

Related issue number

Closes #2279

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 0% with 8 lines in your changes are missing coverage. Please review.

Project coverage is 50.03%. Comparing base (a4c1d58) to head (911ec11).
Report is 1 commits behind head on main.

Files	Patch %	Lines
autogen/agentchat/contrib/agent_optimizer.py	0.00%	8 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2299       +/-   ##
===========================================
+ Coverage   38.16%   50.03%   +11.87%     
===========================================
  Files          78       78               
  Lines        7861     7868        +7     
  Branches     1681     1821      +140     
===========================================
+ Hits         3000     3937      +937     
+ Misses       4611     3600     -1011     
- Partials      250      331       +81     

Flag	Coverage Δ
unittest	14.20% <0.00%> (?)
unittests	49.00% <0.00%> (+10.85%)	⬆️

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[DLWCMD]

Thanks for addressing this issue. However, where can I find information on using this revised code. I see references to llm_config and ao_config_list. Thanks.