 BUG/ENH: Handle AmbiguousTimeError in date rounding
- [x] closes #18946
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Discussed in #22560, adding an `ambiguous` argument to `round`, `ceil` and `floor` so the user can dictate how to handle rounding timestamps that land on ambiguous times.
