ENH: Support NamedAggs in kwargs in Rolling/Expanding/EWM agg method
- [x] closes #28333 
- [x] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [x] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [x] Added an entry in the latest `doc/source/whatsnew/v3.0.0.rst` file if fixing a bug or adding a new feature.

Currently, `Rolling.agg()`, `Expanding.agg()` and `ExponentialMovingWindow.agg()` methods don't support specifying `NamedAgg`s through `**kwargs`, while `Groupby.agg()` does. This change fixes that.
Thanks @snitish 