add a 'prelim_sort' option to JSON faceting

As discusssed in SOLR-9480 ...

Similar to how the rerank request param allows people to collect & score documents using a "cheap" query, and then re-score the top N using a ore expensive query, I think it would be handy if JSON Facets supported a resort option that could be used on any FacetRequestSorted instance right along side the sort param, using the same JSON syntax, so that clients could have Solr internaly sort all the facet buckets by something simple (like count) and then "Re-Sort" the top N=limit (or maybe ( N=limit+overrequest ?) using a more expensive function like skg()