Added an agent description field distinct from the system_message. (#736)

* Added an agent description field that can be distinct from the system_message, and be used to for orchestration (e.g., GroupChatManager, etc.)

* Added debugging.

* Moved default descriptions to constants.

* Fixed conditions under which the assistant uses the default description.

* Removed debugging.

* Updated GroupChat prompt.

* Re-added debugging.

* Removed double [[ ]].

* Another update to GroupSelection prompt.

* Changed 'people' to 'participants' since agents are not people.

* Changed 'role' to 'name'

* Removed debugging statements.

* Restored the default prompt. Created a contrib class with new prompt.

* Fixed documentation.

* Removed broken link.

* Fixed a warning message.

* Removed GroupChatModerator contrib. Will re-add in another PR

* Resolving comment.

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