Adjust retriever's (rrf) rank_window requirements with pagination.

[benwtrent]

Description

Right now, for pagination to work at all and be consistent, we require gathering all the search results that will be ranked in total. Meaning rank_window must be larger than from+size and we then rank the set.

I wonder if we can improve this. It seems there is some performance to be gained. It gets tricky with RRF as its possible that we see a document later in the rank window that wasn't present before in the early pages, thus boosting it and reordering previous pages and subsequent pages.

So, this won't be simple, but I think we can get through the ordering weirdness by faking the ordering in later pages to keep things consistent (e.g. pretend we have seen pages before and recognize we are incorrectly reordering)

[elasticsearchmachine]

Pinging @elastic/es-search-relevance (Team:Search Relevance)