Export arrow2

[toddtreece]

Why

I attempted to use downcast_ref with the appropriate arrow2 types, but since my crate's arrow2 dependency didn't match the plugin SDK's arrow2 dependency, downcast_ref was failing.

frame.values().as_any().downcast_ref::<PrimitiveArray<i64>>();

What

It seems like the only solution was to export the arrow2 so that other crates can access frame values. Not sure if there's a better approach here.

[sd2k]

Bleh, bit annoying but I think you're right, it's the best way of ensuring compatibility (at least until arrow2 releases a long term 1.0). Added a suggestion for a comment to explain/suggest usage of the re-export.