Create endpoint for cross-validation

[wochinge]

Create an endpoint for running a cross-validation as described in the implementation proposal for intent insights here.

Todos:

• Update API spec
• add endpoint to run cross-validation
  • by default it's a synchronous call and the response / error is returned in the response payload
  • If a callback URL is passed as query param, then the server should return immediately. The result (and potential error!) of the cross-validation is passed to the caller by calling the callback
  • We need to make sure that the synchronous cross-validation doesn't block other calls. We can e.g. do this by using run_in_executor as we do it for the training
• consider adding a start / end datetime to the cross-validation result. This would be useful for Rasa X (especially when reading this files from disk in local mode), to see on which data the cross-validation results are based upon