Summary:
This diff adds logic to check whether vector index contains all entries from indexed table.

**Upgrade/Rollback safety:** Added RPC function that used only from test.
Jira: DB-15484

Test Plan: PgBackfillIndexTest.VectorIndex

Reviewers: arybochkin

Reviewed By: arybochkin

Subscribers: ybase, yql

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D42077