Support downcasting of nullable arrays
- [ ] closes #xxxx
- [x] tests added / passed
- [x] passes `black pandas`
- [x] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [x] whatsnew entry

Addressing GH #33013. Supports downcasting of nullable dtypes by transferring elements in extension array to ndarray to avoid TypeError thrown by np.allclose. Enhancement documented on v1.1.0 in the "other enhancements" section. Unit test written in test_to_numeric.
