API endpoint for previewing composed result of an index template v2

[cjcenizal]

Users will want to know what the final result of an index template will be once all of its component templates, mappings, settings, and aliases have been applied. This will be mostly useful during the creation process, so that users can have confidence that an index template they're building will behave as expected.

[elasticmachine]

Pinging @elastic/es-ui (:ES-UI)

[cjcenizal]

Related to #55686. The difference between that API and the request in this issue is that I'd like to be able to test out a single index template and see how its config (including its component templates) gets resolved. I think the linked simulate API could address this by making the index_name param optional, so you could just omit it and only get back the resolved config for the provided body.

Per @dakrone a work-around from the UI side would be to simulate with an index name (e.g. some hash) and change the template to have an index pattern with the same hash so that it will be the only template to match the index name.

[elasticmachine]

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

[dakrone]

Relates to #56390