Allow user to pass in a customized speaker selection method (#1791)

* init PR

* update

* update code check

* update

* update

* update

* update

* Test the ability to have agents a,u,t,o,g,e,n speak in turn.

* update

* update

* update

* Evidence that groupchat not terminating because of the TERMINATE substring.

* Raising NoEligibleSpeakerException allows graceful exit before max turns

* update

* To confirm with author that custom function is meant to override graph constraints

* Confirmed the expected test behaviour with author

* Update autogen/agentchat/groupchat.py

* update

* update

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Co-authored-by: Joshua Kim <Joshua@spectdata.com>
Co-authored-by: Qingyun Wu <qingyun0327@gmail.com>