Separate openai assistant related config items from llm_config

[IANTHEREAL]

Why are these changes needed?

openai assistant related config in llm_config will cause some function not working, like

import logging
import os

from autogen import AssistantAgent, UserProxyAgent, config_list_from_json
from autogen.agentchat.contrib.gpt_assistant_agent import GPTAssistantAgent

logger = logging.getLogger(__name__)
logger.setLevel(logging.WARNING)

assistant_id = os.environ.get("ASSISTANT_ID", None)

config_list = config_list_from_json("OAI_CONFIG_LIST", filter_dict={"tags": ["assistant"]})
llm_config = {"config_list": config_list, "assistant_id": assistant_id}

gpt_assistant = GPTAssistantAgent(
    name="assistant", instructions=AssistantAgent.DEFAULT_SYSTEM_MESSAGE, llm_config=llm_config
)

user_proxy = UserProxyAgent(
    name="user_proxy",
    code_execution_config={
        "work_dir": "coding",
        "use_docker": False,
    },  # Please set use_docker=True if docker is available to run the generated code. Using docker is safer than running the generated code directly.
    is_termination_msg=lambda msg: "TERMINATE" in msg["content"],
    human_input_mode="NEVER",
    max_consecutive_auto_reply=1,
)
result = user_proxy.initiate_chat(gpt_assistant, message="Print hello world", summary_method="reflection_with_llm")
print(result.summary)

Related issue number

Closes #1805

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.

[IANTHEREAL]

duplicate with #1964

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 83.33333% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 66.91%. Comparing base (4429d4d) to head (5ceaa9e).

Files	Patch %	Lines
autogen/agentchat/contrib/gpt_assistant_agent.py	83.33%	2 Missing and 2 partials ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2037       +/-   ##
===========================================
+ Coverage   36.99%   66.91%   +29.92%     
===========================================
  Files          66       66               
  Lines        7015     7031       +16     
  Branches     1534     1666      +132     
===========================================
+ Hits         2595     4705     +2110     
+ Misses       4194     1892     -2302     
- Partials      226      434      +208     

Flag	Coverage Δ
unittests	66.83% <83.33%> (+29.84%)	⬆️

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