Support filtering on group-is-null in filter2 API

[asfimport]

Currently filters only apply to primitive (leaf node) columns. But given the column "a.b.c.d.e.f" it seems reasonable to want to filter on "a.b.c" != null or something like that.

Reporter: Alex Levenson / @isnotinvain

_{Note: This issue was originally created as PARQUET-43. Please see the migration documentation for further details.}