Adds a SocietyOfMindAgent that presents as a single agent, but runs GroupChat as an inner-monologue (#890)

* Added a GroupChatRepresentative class that uses group chat as an inner-monologue.

* Renamed GroupChatRepresentative to SocietyOfMindAgent

* Added tests for the society of mind agent.

* Don't force message attribution -- the defaults are correct.

* Add names to external messages.

* Added some comments to document code, per request.

* Added some comments to document code, per request.

* Added a Notebook example, and the ability to use a prompt as the response_preparer.

* Protect inner chat with try-catch

* Fixed typo.

---------

Co-authored-by: Chi Wang <wang.chi@microsoft.com>