Confirmation

• I confirm that I am a maintainer and so can use this template. If I am not, I understand this issue will be closed and I will be asked to use a different template.

Issue body

For BaseGroupChat, support a public method for getting all the messages happened so far via an async send_message to the group chat manager agent. This will require a new GroupChatGetThread RPC event type, and handled by the BaseGroupChatManager.