ENH: Please provide `register_groupBy_accessor`
Thanks for providing the option to create custom accessors via  [pandas.api.extensions](https://pandas.pydata.org/pandas-docs/stable/development/extending.html). This is prefereable method to extend pandas dataframes over inheritance/sub-classing. Currently, this supports these: dataframe, series, index.

Please extend this support for [groupBy](https://pandas.pydata.org/pandas-docs/stable/reference/groupby.html) objects.