ENH: Add numba engine to groupby.aggregate
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

In the same spirit of #31845 and #32854, adding `engine` and `engine_kwargs` arguments to `groupby.aggreate`.

This PR has some functionality that is waiting to be merged in #32854