[ML] adding for_export flag for ml plugin GET resource APIs (#63092)

This adds the new `for_export` flag to the following APIs:

- GET _ml/anomaly_detection/<job_id>
- GET _ml/datafeeds/<datafeed_id>
- GET _ml/data_frame/analytics/<analytics_id>

The flag is designed for cloning or exporting configuration objects to later be put into the same cluster or a separate cluster. 

The following fields are not returned in the objects:

- any field that is not user settable (e.g. version, create_time)
- any field that is a calculated default value (e.g. datafeed chunking_config)
- any field that would effectively require changing to be of use (e.g. datafeed job_id)
- any field that is automatically set via another Elastic stack process (e.g. anomaly job custom_settings.created_by)


closes https://github.com/elastic/elasticsearch/issues/63055

docs/java-rest/high-level/ml/get-data-frame-analytics.asciidoc

@@ -21,6 +21,9 @@ IDs, or the special wildcard `_all` to get all {dfanalytics-jobs}.
 include-tagged::{doc-tests-file}[{api}-request]
 --------------------------------------------------
 <1> Constructing a new GET request referencing an existing {dfanalytics-job}
+<2> Optional boolean value for requesting the {dfanalytics-job} in a format that can
+then be put into another cluster. Certain fields that can only be set when
+the {dfanalytics-job} is created are removed.
 
 include::../execution.asciidoc[]
 

docs/java-rest/high-level/ml/get-datafeed.asciidoc

@@ -25,6 +25,9 @@ include-tagged::{doc-tests-file}[{api}-request]
 contain wildcards.
 <2> Whether to ignore if a wildcard expression matches no datafeeds.
  (This includes `_all` string or when no datafeeds have been specified).
+<3> Optional boolean value for requesting the datafeed in a format that can
+then be put into another cluster. Certain fields that can only be set when
+the datafeed is created are removed.
 
 [id="{upid}-{api}-response"]
 ==== Get datafeed response

docs/java-rest/high-level/ml/get-job.asciidoc

@@ -25,6 +25,9 @@ include-tagged::{doc-tests-file}[{api}-request]
 wildcards.
 <2> Whether to ignore if a wildcard expression matches no {anomaly-jobs}.
  (This includes `_all` string or when no jobs have been specified).
+<3> Optional boolean value for requesting the job in a format that can
+then be put into another cluster. Certain fields that can only be set when
+the job is created are removed.
 
 [id="{upid}-{api}-response"]
 ==== Get {anomaly-jobs} response