Users cannot select chat-engine in the chat window with non-logged in mode

[iamxy]

In non-logged in mode, users cannot select chat-engine in the chat window. However, they can switch in administrator login mode. Hope this can be fixed :)

[iamxy]

How about adding an option in the configuration UI of the admin panel: "whether users can switch 'chat-engines' in anonymous mode", taking into account both tidb.ai and local deployment.

[Mini256]

Added is_public option for ChatEngine in pull request #692, and the non-logged users can switch the public chat engines