Highlighting support (leverage Elasticsearch highlighting)

Attempted to redo this patch for version 7 and drupal 9.5

Re-roll the patch in #10. It was giving me fatal error because the changes for the new src/Utility.php file was missing.

I've created a patch for ElasticSearch 8, that leverages Search API's API a little better, and supports a handful more options. Let's see if we can get it into the 8.0.x branch first, then backport it.

I've created merge request !73. Reviews are welcome!

This seems like something we could also contribute to Search API OpenSearch → !

(updated the issue summary)

Updated the issue summary, for parity with search_api_opensearch's issue ✨ Support OpenSearch server highlighting Active .

Moving to Needs Work, because I think I'd like to add another test, this time against the ElasticSearch environment in CI.