Summary:
In order to provide vector index backfill consistency we should use proper read time while reading indexed table during backfill.
It is possible to use add table operation hybrid time for this purpose, so all rows written/committed before this time will be added to index during backfill. While all rows written after this time, will be inserted to the index while intents being applied.
Jira: DB-15374
Original commit: 8aae4a946a333ce24cfd3d7b574cdd44ebccfa80 / D41707

Test Plan: PgIndexBackfillTest.VectorIndex

Reviewers: arybochkin, slingam

Reviewed By: slingam

Subscribers: yql, ybase, mihnea, slingam, smishra, jason

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D42078