ENH: IntegerArray.astype(dt64)
- [x] closes #32435
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Let's us de-kludge to_datetime code, getting rid of another _ndarray_values usage.