* Use V2 index templates during index creation
This commit changes our index creation code to use (and favor!) V2 index templates during index
creation. The creation precedence goes like so, in order of precedence:
- Existing source `IndexMetadata` - for example, when recovering from a peer or a shrink/split/clone
where index templates should not be applied
- A matching V2 index template, if one is found
- When a V2 template is found, all component templates (in the `composed_of` field) are applied
in the order that they appear, with the index template having the 2nd highest precedence (the
create index request always has the top priority when it comes to index settings)
- All matching V1 templates (the old style)
This also adds index template validation when `PUT`-ing a new v2 index template (because this was
required) and ensures that all index and component templates specify *no* top-level mapping type (it
is automatically added when the template is added to the cluster state).
This does not yet implement fine-grained component template merging of mappings, where we favor
merging only a single field's configuration, that will be done in subsequent work.
This also keeps the existing hidden index behavior present for v1 templates, where a hidden index
will match v2 index templates unless they are global (`*`) templates.
Relates to #53101
814c248819