ENH: Add `method='table'` for EWM.mean
- [x] tests added / passed
- [x] Ensure all linting tests pass, see [here](https://pandas.pydata.org/pandas-docs/dev/development/contributing.html#code-standards) for how to run them
- [x] whatsnew entry
close #42273 
Since this only works for `engine='numba'` and for `mean` would it not be better to wrap this simply into `mean(engine='numba-table')` or the like?

Or, is the plan to try and expand this functionality more generally and therefore your way is better? Yes, the hope is to eventually enable `method="table"` for other `EWM` operations (`var`, `std`) in the future with numba.

This also mirrors the API we have for `rolling` and `expanding` thanks @mroeschke 