Adds ML start and stop trained model deployment specifications

[lcawl]

Relates to elastic/elasticsearch#73660

Adds specifications for the start trained model deployment API and stop trained model deployment APII based on the documentation and specs (ml.stop_trained_model_deployment.json, ml.start_trained_model_deployment)

[lcawl]

I wasn't entirely confident about how to represent the routing_table object in the start trained model deployment specification, but I've given it a go with the dictionary type in c6714b0

[dimitris-athanasiou]

The response of the stop API should be like:

{
  "stopped" : true
}