[ENH] Allow pad, backfill and cumcount in groupby.transform
- [x] closes #31269
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Worth noting that the added aliases `pad` -> `ffill` and `backfill` -> `bfill` in `pandas/core/generic.py` are not necessary for resolving issue #31269.

However, the same thing was done in `pandas/core/groupby/groupby.py` so I thought it'd be nice to add these 2 aliases too. Besides, it makes the test `test_transform_transformation_func` simpler because of the added consistency.

Do let me know if you guys think this is inappropriate. I will remove them.
 @WillAyd  btw should I mention the 2 new aliases in whatsnew? I don't have an issue number to tag it to though. Should I create an issue for that? ping Thanks @fujiaxiang another great PR - keep at it!