Adds a "node" field to the response from the following endpoints:
1. Open anomaly detection job
2. Start datafeed
3. Start data frame analytics job
If the job or datafeed is assigned to a node immediately then
this field will return the ID of that node.
In the case where a job or datafeed is opened or started lazily
the node field will contain an empty string. Clients that want
to test whether a job or datafeed was opened or started lazily
can therefore check for this.
Backport of #55473
* HLRC: ML Add preview datafeed api
* Changing deprecation handling for parser
* Removing some duplication in docs, will address other APIs in another PR
* HLRC: ML Cleanup docs
* updating get datafeed stats docs
David Roberts
Lisa Cawley
Benjamin Trent