Fix for http client

[AbdurNawaz]

Why are these changes needed?

When you add the parameter http_client (which is typically an instance of httpx.Client or httpx.AsyncClient) to llm_config, the method copy.deepcopy throws an error as it won't be able to pickle the client object. This is a fix to skip the copying if an http_client parameter is passed.

Code to reproduce:

import httpx

http_client = httpx.Client()
                          
mixtral_config_list = [                                                                                                
    {
        "base_url": "OPENAI_API_BASE_URL"
        "api_key": "OPENAI_API_KEY",                                                                                       
        "http_client": http_client,
    }
]

#the below line will throw an error
assistant = AssistantAgent("assistant", llm_config={"config_list": config_list})

Related issue number

#2412

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.

[ekzhu]

Thanks for the PR. Given @Gr3atWh173 's comment: #2412 (comment)
I think instead of checking for http_client key, we should try and raise the error but append a more meaningful error message saying please implement the value class with __deepcopy__ method to support deepcopy.

[AbdurNawaz]

@microsoft-github-policy-service agree

[AbdurNawaz]

@ekzhu agreed - I've added the necessary changes and updated the docs. Please review

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 50.00000% with 2 lines in your changes are missing coverage. Please review.

Project coverage is 25.06%. Comparing base (ded2d61) to head (7ad33ce).
Report is 33 commits behind head on main.

Files	Patch %	Lines
autogen/agentchat/conversable_agent.py	50.00%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2579      +/-   ##
==========================================
- Coverage   33.33%   25.06%   -8.27%     
==========================================
  Files          83       85       +2     
  Lines        8636     9011     +375     
  Branches     1835     2069     +234     
==========================================
- Hits         2879     2259     -620     
- Misses       5516     6491     +975     
- Partials      241      261      +20     

Flag	Coverage Δ
unittest	12.38% <0.00%> (?)
unittests	24.62% <50.00%> (-8.72%)	⬇️

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