[FEA] Add support for equivalence of Pandas 'unstack' on Series

**Is your feature request related to a problem? Please describe.**
Hi,

While porting some code from Pandas to cuDF, I have noticed that cuDF series do not support `unstack` method.
As an additional request, It would be great if `fill_values` could be supported in both `cudf.DataFrame.unstack` and `cudf.Series.unstack` methods. Thanks!

**Describe the solution you'd like**
To have that method supported in cuDF series.

**Describe alternatives you've considered**
Creating a cuDF DataFrame from the cuDF series, use `unstack` on the DataFrame.