Restore configuration of the Similarity in the Lucene backend

Description

https://docs.jboss.org/hibernate/search/5.11/reference/en-US/html_single/#section-custom-similarity

Let's restore this setting, but allow it to be defined per backend only.
We will need to explicitly disallow searches across Lucene backends.

IMPORTANT: BM25, the default in Elasticsearch, previously did not use the Similarity interface. There were some problems due to it not being implementable with the Similarity interface. Let’s check, and make sure it’s either the default, or can now be set as a Similarity.

Environment

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Assignee

Unassigned

Reporter

Yoann Rodière

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Pull Request

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Components

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Priority

Major
Configure