Add support for composable index templates [DATAES-885]

[spring-projects-issues]

sothawo opened DATAES-885 and commented

Elasticsearch 7.8 introduces composable index templates. These should be support in Spring Data Elasticsearch as well

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No further details from DATAES-885

[sothawo]

Note: legacy index template API is deprecated as of Elasticsearch 7.13.0

[sothawo]

Started working on that, but it seems that the necessary API calls are missing from the client (elastic/elasticsearch-java#520)