Add timetz attribute to DatetimeIndex
- [x] closes #21358 
- [x] tests added / passed
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

I only added timetz to DatetimeIndex because there was already a timetz method in Timestamp that did essentially the same thing. If it is still worth adding to Timestamp though, I can certainly do that.
