Summary:
Rename vector library to vector_index library.
Also rename vectorindex namespace to vector_index namespace.
Jira: DB-13694

Test Plan: Jenkins

Reviewers: mbautin, aleksandr.ponomarenko, arybochkin

Reviewed By: arybochkin

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D39480