Summary:
Initial implementation of Vector LSM.

Introduces Vector LSM interface and some basic stubs.
Implements insert of vector batch in multiple thread.
Search implementation is incomplete.
Jira: DB-12907

Test Plan: Jenkins

Reviewers: mbautin, xCluster, hsunder

Reviewed By: mbautin

Subscribers: ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D37877