This adds support for a `dry_run` parameter for the
`_ilm/migrate_to_data_tiers` API. This defaults to `false`, but when
configured to `true` it will simulate the migration of elasticsearch
entities to data tiers based routing, returning the entites that need to
be updated (indices, ILM policies and the legacy index template that'd
be deleted, if any was configured in the request).
This adds the _ilm/migrate_to_data_tiers API to expose the service for
migrating the elasticsearch abstractions (indices, ILM policies and an
optional legacy template to delete) to data tiers routing allocation
(away from custom node attributes)
Andrei Dan