Provide Group By Context To Aggregation

[joe-l-bright]

I would like to discuss adding 'existingGroupBy' as a param to the aggregation...

https://github.com/tannerlinsley/react-table/blob/d2acfc423593389f8f7b974c6f544ecf683f0724/src/plugin-hooks/useGroupBy.js#L246

We have a use case where aggregations are determined based on Groupings.

e.g. If I group by Age, I would like to aggregate Height with Average, but if I group by City, I would like to aggregate Height with Mean.

If we pass GroupBy into the function, I could then create a mapping of group to aggregate and look that up before rendering.

Please advise of any existing solutions as well! Thanks.