Summary:
This diff continues work started at D39478
- implement iterator over vectors for usearch wrapper
- introduce tests for merging usearch indexes
- implement  GetVector for usearch wrapper, and for hnswlib wrapper
- introduce MaxVectors method
- make member_iterator_t public in usearch index_dence

Jira:DB-12962

Test Plan: merge-vector-indexes

Reviewers: sergei, arybochkin

Reviewed By: sergei

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D40253