cuvs-java: Evaluate adding a query mapping function (avoids reifying a mapping list)

[ChrisHegarty]

This issue has been filed to evaluate modifying the Builder withMapping(List<Integer> mapping) API, so that it does not require to reify a concrete list of map ids.

For example:

List<Integer> map =...

var query = new CagraQuery.Builder()
        .withMapping(map)

From my understanding a function of int -> int is sufficient. Maybe an additional size(). For Lucene this would work better than reifying the ordinal to doc mapping as a list.

e.g.

Builder withMapping(MappingFunction ordToDocFunc)

@FunctionalInterface
public interface MappingFunction {
  /** The given a vector ordinal returns a docId */
  int apply(int value);

  int size(); // if needed
}