[FEATURE] Implement IndexTemplate related methods in RestIndexTemplate

[ymombrun]

Is your feature request related to a problem?

Since version 5.1 of spring-data-elasticsearch, various methods from IndexOperation interface have been added and a few other have been marked as deprecated. For instance to check if a template exist, deprecated method is existsTemplate while methods existsComponentTemplate and existsIndexTemplate have been added.
Unfortunately when trying to remove any deprecated call from my project code relying on this, I got a UnsupportedOperationException. Indeed inside RestIndexTemplate (from spring-data-opensearch) there are 8 methods throwing UnsupportedOperationException("not implemented"); all related to index and component template CRUD.

What solution would you like?

I'd like that the new component and index template related methods were implemented so that I can use them and not relying on deprecated API in my project (which triggers warning in my CI that I can't solve yet).

What alternatives have you considered?

I keep relying on the deprecated API, but then when spring-data-elasticsearch will remove the deprecated methods from the interface (in 5.2 for instance).

Do you have any additional context?

The elastic search specific implementation from spring-data-elasticsearch is (in release 5.1.4) at the same status, methods not implemented.

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@ymombrun Want to try to implement this?