Add dedicated simulation API for simulating a template without an index #56390
Description
Relates to #53101
We currently have an /_index_template/_simulate_index/{index} API that allows simulating (with our without a body) the resolved body for a given index. We should add a similar API for simulating only a template without an index, something like:
POST /_index_template/_simulate/{template-id}
Where the template-id is optional, and if not specified, then the body must be present with a valid template definition.
Relates to #56295