This enhancement adds a new abstraction to the _search API called "retriever." A
retriever is something that returns top hits. This adds three initial retrievers called
"standard", "knn", and "rrf". The retrievers use a parser-only approach where they
are parsed and then translated into a SearchSourceBuilder to execute the actual
search.
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Co-authored-by: Mayya Sharipova <mayya.sharipova@elastic.co>
