Add query matching keywords in elasticsearch response.

[fmassot]

This PR adds the list of keywords the query should match in the elasticsearch response for use in the Grafana plugin.
I consider this PR good enough for use in Grafana even though we have the following caveats:

• The Grafana plugin expects a list of keywords for the query, this is less accurate than a tantivy snippet returned for each document.
• We don't use tokenizers to deduce the list of keywords. This means that a document can match, but no keyword will be highlighted in Grafana.