API: change .resample to be a groupby-like API
similar to #11603 

this would transform:

`s.resample('D',how='max')`

to

`s.resample('D').max()`

This would be a breaking API change, as the default is `how='mean'`, meaning, that `s.resample('D')` returns the `mean` of the resampled data. However it would be visible at the very least and not simply change working code.

This would bring `.resample` (which is just a groupby type operation under the hood anyhow) into the API syntax for `.groupby` and `.rolling` et. al.

Furthermore this would allow geitem / aggregate type operations with minimal effort
e.g.

`s.resample('D').agg(['min','max'])`
