introduce support in KnnVectorQuery for getters

[alessandrobenedetti]

Description

Knn Queries are locked currently, it would be beneficial for applications using them to have access to getters and setters.
An example is how filter queries are managed in Apache Solr:
the processing of pre-filters and post-filters could benefit from opening up the access to such variables.
Especially the pre-filter support introduced in Solr 9.1 could get great benefits from being able to set the filter, after the query has been parsed.
See:
apache/solr#1245