Center window
- [x] closes #20012
- [ ] tests added / passed
- [ ] passes `black pandas`
- [ ] passes `git diff upstream/master -u -- "*.py" | flake8 --diff`
- [ ] whatsnew entry

Added center functionality for `VariableWindowIndexer`. Note - I am unsure if the NotImplementedError in lines 1966-1969 in rolling.py still correctly raises an error for offset based windows.