Don't specify `AbstractArray` for column types in DataFrame `rcopy`

[asinghvi17]

This allows other packages to hook into the conversion system when loading R DataFrames.

My usecase here is to provide seamless interop between R and Julia for sf dataframes, so people can take advantage of all of R's tooling without having to re-implement it in Julia.

An example of the use is that by defining rcopytype and rcopy for sfc_MULTIPOLYGON, which is a simple features collection of multipolygons, I can get those geometries converted to the relevant GeoInterface.jl geometry representation for free.