Support function_call in autogen/agent

[yiranwu0]

Why are these changes needed?

Preliminary designs to add the new function_call feature into the agent framework.

Related issue number

See #1083 for tasks.

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.

[sonichi]

merge with main and simplify the notebook.
add test.

[yiranwu0]

Some Notes:

1. In UserProxyAgent.receive, I added back the line to convert the message to a dict. This is to make the type of the message consistent to avoid keep checking the type. Thus, We may define is_termination_msg to always take in a dict. The type of message passed to auto_reply should also be a dict.

2. UserProxyAgent now allows the function calls calling to a class method, and passing in predefined arguments, or even update these arguments if needed. The current design is to ask the user to pass in a dict. More details in documentation of UserProxyAgent.