NEW FEATURES: ROLLUP, CUBE, and GROUPING SETS

[iki77]

Most of my use cases for reporting need drill-downs, it would be nice to use ROLLUP, CUBE, and GROUPING SETS directly instead of my commonly used backend, Pyspark.

[cpcloud]

I've got a stalled PR for this that is pretty close, I just haven't had a moment to wrap it up yet.

Since it's not merged yet (and even if it were!) it would be great to get your feedback on the API. I will rebase it so it's on current main.