[ML] add new snapshot upgrader API for upgrading older snapshots (#64665)

This new API provides a way for users to upgrade their own anomaly job
model snapshots.

To upgrade a snapshot the following is done:
- Open a native process given the job id and the desired snapshot id
- load the snapshot to the process
- write the snapshot again from the native task (now updated via the
  native process)

relates #64154

docs/java-rest/high-level/ml/upgrade-job-model-snapshot.asciidoc

@@ -0,0 +1,44 @@
+--
+:api: upgrade-job-model-snapshot
+:request: UpgradeJobModelSnapshotRequest
+:response: UpgradeJobModelSnapshotResponse
+--
+[role="xpack"]
+
+[id="{upid}-{api}"]
+=== Upgrade job snapshot API
+
+Upgrades a previously stored {ml} model snapshot to the
+current major version.
+It accepts an +{request}+ object and responds
+with an +{response}+ object.
+
+[id="{upid}-{api}-request"]
+==== Upgrade job snapshots request
+
+A +{request}+ requires the following arguments:
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-request]
+--------------------------------------------------
+<1> The job that owns the snapshot
+<2> The snapshot id to upgrade
+<3> The time out of the request. When `wait_for_completion` is `false` the timeout value is
+    applied to the time it takes for the task to be assigned to a node. When `wait_for_completion`
+    is `true` this timeout applies for the whole upgrade process. The default value is `30m`
+<4> When true, this causes the request to not return until the upgrade is complete.
+    Otherwise, the response returns as soon as the task
+    is assigned to a node. The default is false.
+
+[id="{upid}-{api}-response"]
+==== Upgrade job snapshot response
+
+A +{response}+ contains information on if the request was completed and its assigned node.
+
+["source","java",subs="attributes,callouts,macros"]
+--------------------------------------------------
+include-tagged::{doc-tests-file}[{api}-response]
+--------------------------------------------------
+<1> Was the upgrade completed
+<2> What is the assigned node if the task is not completed

docs/java-rest/high-level/supported-apis.asciidoc

@@ -344,6 +344,7 @@ The Java High Level REST Client supports the following {ml} APIs:
 * <<{upid}-update-datafeed>>
 * <<{upid}-update-filter>>
 * <<{upid}-update-model-snapshot>>
+* <<{upid}-upgrade-job-model-snapshot>>
 
 // CLOSE
 include::ml/close-job.asciidoc[]
@@ -421,6 +422,7 @@ include::ml/update-data-frame-analytics.asciidoc[]
 include::ml/update-datafeed.asciidoc[]
 include::ml/update-filter.asciidoc[]
 include::ml/update-model-snapshot.asciidoc[]
+include::ml/upgrade-job-model-snapshot.asciidoc[]
 
 == Migration APIs