Enable multi column group by for Dataframe

### System information

- **OS version/distro**:windows
- **.NET Version (eg., dotnet --info)**: .net core 5.0

### Issue
I am using the Dataframe object from Microsoft.Data.Analytics, currently GroupBy only allows to group by only one column. There isn’t an option to group by multiple columns.
- **What did you do?**
- created a Dataframe with a few columns
- **What happened?**
- tried to use group by multiple columns
- **What did you expect?**
- Expected a method that took a list of columns names to group the Dataframe by

### Source code / logs

Please paste or attach the code or logs or traces that would be helpful to diagnose the issue you are reporting.