`SelectorGroupChat` to add `model_context` parameter and make the group chat manager use the model context for selecting speaker

[ekzhu]

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Issue body

This makes it possible to customize the selector and better context management.

Related #6292

[Ethan0456]

Hi @ekzhu, I'd like to give this a try. Could you please assign this issue to me?

[ekzhu]

Sure, please keep in mind of concurrent work in #6303 and #6291

[appuk45]

When will this feature will release?

[Ethan0456]

@ekzhu This issue was addressed in #6330, which has been merged. I believe we can close it now.