[7.x] Implement framework for migrating system indices (#78951) #79525
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Backport of #78951.
This PR adds a framework for migrating system indices as necessary prior
to Elasticsearch upgrades. This framework uses REST APIs added in
another commit:
This API, which gets the status of "features" (plugins which own system
indices) with regards to whether they need to be upgraded or not. As of
this PR, this API also reports errors encountered while migrating system
indices alongside the index that was being processed when this occurred.
As an example of this error reporting:
This API starts the migration process. The API for this has no changes,
but when called, any system indices which need to be migrated will be
migrated, with status information stored in the cluster state for later
use by the GET _migration/system_features API.