This PR introduces a new query called `combined_fields` for searching multiple
text fields. It takes a term-centric view, first analyzing the query string
into individual terms, then searching for each term any of the fields as though
they were one combined field. It is based on Lucene's `CombinedFieldQuery`,
which takes a principled approach to scoring based on the BM25F formula.
This query provides an alternative to the `cross_fields` `multi_match` mode. It
has simpler behavior and a more robust approach to scoring.
Addresses #41106.
