Groupchat send introductions (#961)

* Allow the GroupChatManager to send introductions.

* Fixed function name.

* Added test cases for sending introductions.

* Trying to sort out why remote pytest is failing.

* Fixed broken plugin behavior.

* Update autogen/agentchat/groupchat.py

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

* Updated as per Chi's suggestions.

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Co-authored-by: Chi Wang <wang.chi@microsoft.com>
Co-authored-by: Eric Zhu <ekzhu@users.noreply.github.com>