PERF: fix #32976 slow group by for categorical columns
Aggregate categorical codes with fast cython aggregation for select `how` operations. Added new ASV benchmark copied from 32976 indicating > 99% improvement in performance for this case.

- [x] closes #32976
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
