ENH/BUG: Add is_dst method to DatetimeIndex and Timestamp to solve AmbiguousTimeError
- [x] closes #18885
- [x] closes #18946
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

The issues above required a new method to keep track if each timestamp in the `DatetimeIndex` was previously in daylight savings time in order to pass into `tz_localize(..., ambiguous=...)`

Therefore, added a `is_dst` method to `DatetimeIndex` and `Timestamp`.