Serialization for the functions, fixed issues in agentchat selector_func and candidate_func serialization

[izam-mohammed]

Why are these changes needed?

This PR adds a new IndividualFunction module that allows functions to be serializable for better persistence and restoration in GroupChat. It updates the selector and candidate functions in SelectorGroupChat to use this new implementation, which supports both synchronous and asynchronous functions through a unified interface. The PR also adds a utility to automatically detect imports used by functions, making the serialization process more robust.

Checks

• I've included any doc changes needed for https://microsoft.github.io/autogen/. See https://github.com/microsoft/autogen/blob/main/CONTRIBUTING.md 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.

[izam-mohammed]

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[ekzhu]

Thanks! Can we also have some unit tests for this? Serializing a SelectorGroupChat with the custom functions and deserialize it back, verify that the new one works.

[ekzhu]

Why not use FunctionTool?

[izam-mohammed]

That would be a quick fix. But fundamentally both are different. For data classes and others, this can also be scaled independently. For now, it would work, but I think we don't have to treat every reusable function as a tool. In the case of SelectorGroupChat selector_func is fundamentally just a function, not a tool. if you need it that way, I can do that easily. but I don't think it is better for scalability and other factors.