Support index-only scan for columnar tables

With #4950, we implement index support for columnar tables.

On the other hand, even when `EXPLAIN` output tells that we will do index-only scan, it's never the case since columnar tables don't have the visibility info that postgres is looking for.
For this reason, visibility check in `IndexOnlyNext->VM_ALL_VISIBLE` code-path always returns false and postgres always reads the tuple from the columnar relation.