[FEA] Support for `cudf.SparseDtype` and sparse array

[galipremsagar]

Is your feature request related to a problem? Please describe. This is a feature request for sparse type and sparse column in cudf, these are the pandas counter parts:

https://pandas.pydata.org/docs/reference/api/pandas.arrays.SparseArray.html https://pandas.pydata.org/docs/reference/api/pandas.SparseDtype.html

[wbuchanan]

I would second this request. It's biting me right now on a project where the sparse representation is the only way to maintain all of the data in memory.