Previously, a datafeed and job must already exist for the `_preview` API to work.
With this change, users can get an accurate preview of the data that will be sent to the anomaly detection job
without creating either of them.
closes https://github.com/elastic/elasticsearch/issues/70264
Secondary authorization headers are to be used to facilitate Kibana spaces support + ML jobs/datafeeds.
Now on PUT/Update/Preview datafeed, and PUT data frame analytics the secondary authorization is preferred over the primary (if provided).
closes https://github.com/elastic/elasticsearch/issues/53801
