Replace need for KnnVectorValues.copy() with a dictionary interface

[msokolov]

Description

The idea is to get rid of copy() since it does more than is needed. What's needed is an independent vector accessor over the same values source. Adrien had suggested a dictionary() interface that would take over the random-access API from the values source (or in addition to it?) See discussion in https://github.com/apache/lucene/pull/13779 for more details.