Summary:
Originally `PgSamplePicker` was inherited from `PgSelectIndex` in order to be able to provide ybctids to PgSample for sampling table rows.
Later, D36185 introduced `YbctidProvider` and `SetRequestedYbctids` function specifically for this purpose.

To simplify workflow and remove unnecessary code dependencies this revision updates `PgSamplePicker` to no longer depend on `PgSelectIndex` and just get ybctids from it and pass it to  `SetRequestedYbctids`.
Jira: DB-15151

Test Plan: Jenkins

Reviewers: amartsinchyk, dmitry, telgersma

Reviewed By: amartsinchyk

Subscribers: yql, ybase

Tags: #jenkins-ready

Differential Revision: https://phorge.dev.yugabyte.com/D41516