SOLR-17928: Add efSearch to knnVectorsQuery

[chatman]

https://issues.apache.org/jira/browse/SOLR-17928

Elastic uses num_candidates
Industry generally calls it efSearch

We need a parameter to overfetch:
https://solr.apache.org/guide/solr/latest/query-guide/dense-vector-search.html#knn-query-parser

Elastic:
https://www.elastic.co/search-labs/blog/elasticsearch-knn-and-num-candidates-strategies

[chatman]

https://issues.apache.org/jira/browse/SOLR-17928