Add migrate to data tiers API

[andreidan]

This adds the 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).
The details for how the migration is working are specified in #73689

The API is

POST /_ilm/migrate_to_data_tiers
{
  "legacy_template_to_delete": "template_name",
  "node_attribute": "data"
}

The response will contain the name of the deleted legacy index template
(if any was deleted), a list of ILM policies and a list of indices names containing
the policies and indices that were migrated.

{
  "removed_legacy_template":"catch-all-temaplte",
  "migrated_ilm_policies":["policy-one","policy-two"],
  "migrated_indices":["logs-000001","indexwithdatawarmrouting",".ds-.slm-history-5-2021.04.22-000002"]
}

Relates to #73154

[elasticmachine]

Pinging @elastic/es-core-features (Team:Core/Features)

[andreidan]

@elasticmachine update branch

[andreidan]

@elasticmachine run elasticsearch-ci/part-2

[elasticmachine]

Pinging @elastic/es-docs (Team:Docs)

[elasticmachine]

Pinging @elastic/clients-team (Team:Clients)

[elasticmachine]

Pinging @elastic/kibana-stack-management (Team:Stack Management)

[sethmlarson]

Would this API be better suited in a namespace? Since it appears to be documented nearby ILM APIs perhaps the ilm namespace is a good choice (despite of course making changes to indices/templates as well)?

[jethr0null]

@dakrone @andreidan @sajjadwahmed could you please help us to understand the desired UX/user flows for this change (especially as it relates to any requested UI updates)?

[dakrone]

This LGTM, thanks for working on it Andrei. I do think this should be nested within the /_ilm/ namespace, so POST /_ilm/migrate_to_data_tiers, as I think it fits best there rather than a top-level API. What do you think?

[dakrone]

could you please help us to understand the desired UX/user flows for this change (especially as it relates to any requested UI updates)?

I don't think we currently have plans to expose this as part of the UI (please correct me if I'm mistaken @andreidan), but this would generally be something invoked as a one-off by an administrator or at the behest of support to help migrate without having to manually do the migration.

[andreidan]

@dakrone thanks for the review. Nested API under the /_ilm/ namespace. The API now is _ilm/migrate_to_data_tiers