Fixed Issue Preventing Agg on RollingGroupBy Objects
- [X] closes #15072
- [X] tests added / passed
- [X] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

AFAICT the fact that RollingGroupBy could not use `agg` with a list of functions is simply due to the fact that the GroupByMixing it inherits from could not handle the reduction in dimensions that occurs via the normal aggregation functions.