knn_operator to be applicable _only_ to knn queries

We're changing the behavior here a bit (spec to be updated).

Per elastic - https://www.elastic.co/guide/en/elasticsearch/reference/current/knn-search.html#_combine_approximate_knn_with_other_features , kNN results are always unioned (disjuncted) with the exact search results in a hybrid request. We've somewhat been leaning towards this as well, so we'll change the behavior of knn_operator to be applicable to ONLY the kNN queries.

The search request syntax will remain as it were.