Add ability to switch/handoff conversation between two Rasa endpoints

[rgstephens]

There is an implementation and description of this feature in the Rasa helpdesk example bot docs here.

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[hotzenklotz]

What is the purpose of this PR? Looks like you just copied the Rasa fork? Why not use that directly?

[rgstephens]

I'd like to extend this code and think it is a valuable addition to the original project.