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

Finalizes previous PR https://github.com/pandas-dev/pandas/pull/36097