agg with list of non-aggregating functions
- [x] closes #35490
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Reverts to 1.0.5 behavior while maintaining the bugfix that introduced the "regression". Ideally `agg` would instead raise in these scenarios ([ref](https://github.com/pandas-dev/pandas/issues/35490#issuecomment-672981745)), but that would be an API change. Much of this PR should be reverted once this is done, I've marked such places with TODOs.
