Configurable shards, replicas

Initial implementation, minimally tested.

Thanks for working on this! Since the proposed solution creates data model changes this would require an update hook.
The solution opens few questions:
1. What if user has configured the default number of shards and replicas on elasticsearch.yml index.number_of_replicas, this will be a breaking change for those users.
2. The code (without update path) defaults to 1 for number of replicas and this will cause the health of a single node cluster to always be yellow (instead of green).

One solution could be to use or extend IndexCreatedEvent event to achieve the same on the custom module. Elasticsearch provides quite some index settings (https://www.elastic.co/docs/reference/elasticsearch/index-settings/index...) so I'd say that adding new configurations to Drupal UI should be carefully considered.