ExtensionDtype API for preferred type when converting to NumPy array 
This is coming out of SparseArray. Right now if you have a homogeneous DataFrame of EAs, then doing something like `np.asarray(df)` will always convert to object. https://github.com/pandas-dev/pandas/blob/32a74f19b655f4b0ef198d5ff4e15d409c3670d2/pandas/core/internals/managers.py#L787-L794.

Should we give ExtensionDtype some input on the dtype used here? This would allow for some consistency between `np.asarray(series)` and `np.asarray(homogeneous_dataframe)`.