Json config list, agent refactoring and new notebooks

[sonichi]

Why are these changes needed?

1. provide a utility function to load a config list from json. One can put all the endpoints in a single place and filter when needed.
2. update notebooks.
3. sanitize the handling of config list.
4. handle openml error in the tests. ServerError: server failed with HTTP status code 503 openml/OpenML#1196
5. move is_termination_msg arg from UserProxyAgent to Agent and allow AssistantAgent to terminate the conversation.
6. create an AIUserProxyAgent which inherits UserProxyAgent and can make ai_reply like AssistantAgent.
7. add a notebook for multi-agent multi-user applications.
8. other refactoring of agents.
9. add a notebook example for using IPython to execute code.

Related issue number

close #1139
close #1137
close #1141

Checks

• I've included any doc changes needed for https://microsoft.github.io/FLAML/. See https://microsoft.github.io/FLAML/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.