Arrow selection functions

[wjones127]

• Get object field
• Get array element

Should likely offer two variants:

1. get_variant_field_zc: Return BinaryViewArray so we can re-use the original buffer. This is can be used when the user will immediately use a cast method to extract data.
2. get_variant_field: Return BinaryArray so we can drop the original buffer and keep the narrowed values. This can be used when the original variant data is no longer needed but we will be passing the variant selection to another step.

In DataFusion, we can add an optimization rule to decide which of these to use.