ENH: implement efficient sorting methods for SparseArray (argsort/argmin/argmax)
xref https://github.com/pandas-dev/pandas/pull/27801#discussion_r426017444

`SparseArray` currently does not implement `_values_for_argsort` or `argsort/`argmin`/`argmax` itself, which means it is using the base implementation, which converts the sparse array to a dense numpy array. Room for improvements thus.