[FEA] Support null_equality parameter in cudf::contains

[ttnghia]

The current implementation of cudf::contains(column_view const& haystack, column_view const& needles) doesn't support null_equality option. As such, any null row in the input needles column will result in a null in the corresponding output row. If we want to actually check for contains/existence for null rows from needles, we have to work around somehow.

It is better to support null_equality so we call specify whether a null row from needles will result in a null output row, or will be actually checked for existence in haystack. This is necessary to support calling cudf::contains in semi_anti_join implementation (#11037).

[ttnghia]

Just realize that semi_anti_join can just work around this during building the gather map. So close this.