implement .dt accessor for Index classes
`index.dt` --> `self` for `DatetimeIndex`, `TimedeltaIndex`, `PeriodIndex`.  Raises `AttributeError` for others (identical error to `Series.dt`).

@jorisvandenbossche in the discussion in #17134 you had a concern about tab completion/namespacing.  Is that a sticking point for you?

- [x] closes #17134
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry
