[FEA] concatenate lists together

[revans2]

Is your feature request related to a problem? Please describe.
We would like to be able to support the concat operator in Apache Spark sql for arrays. concat lets you concatenate strings together into a new string. It also lets you concatenate lists of other things together into a new list.

This is very similar to what happens with strings, the main difference is that a list can hold nulls and a string cannot, so we need to make sure that the nulls are honored.

Also if one of the input lists is a null the output list should be a null.

Describe the solution you'd like

I would like an API like the following that would take multiple lists of Something and concatenate them together.

std::unique_ptr<column> concatenate(
  table_view const& lists_columns,
  rmm::mr::device_memory_resource* mr = rmm::mr::get_current_device_resource());

Describe alternatives you've considered
I think I hack something together myself but it would not be pretty.