Add RowCount stat to zone maps for more precise pruning

[xiaoxuandev]

Zone maps currently lack a RowCount stat. This limits certain pruning strategies
that need to compare a count against the total number of rows in a zone.

For example, to falsify is_not_null(x) the ideal condition is:

null_count == row_count => is_not_null is all false

Without RowCount, we have to fall back to is_constant && null_count > 0, which
only catches the subset of cases where the zone is constant.

Adding RowCount would enable more precise pruning for is_not_null and potentially
other expressions that need to reason about total row counts.

[gatesn]

Zone maps currently have a fixed row count, so we could falsify the expression by returning the literal? Although I think i might be a bit fiddly with the final zone that may have fewer rows. We might just need to construct the row count as a RunEndArray.

The other thing to mention is that I'm planning to move zone map statistics over to using arbitrary AggregateFn, so we should definitely be able to define a "dumb" RowCount aggregate function?