ENH: Add VariableForwardWindowIndexer
#### Is your feature request related to a problem?

I would like to use the rolling() method with looking-forward window using a negative `window`.
I don't remember which other pandas function is using this approach of negative window for reversing the position of the window with respect to the actual index, but it is for the user very convenient.
I was convinced it was existing for rolling, but it appears it is not.

#### Describe the solution you'd like

```python
# Definition of a negative offset (this works)
offset = pd.tseries.frequencies.to_offset('-1h')

# Use it in rolling (this does not work)
start = '2020-01-01 08:00'
end = '2020-01-01 12:00'
intervals = pd.period_range(start = start, end=end, freq = '30T')
values = [i for i in range(0, len(intervals))]
ser = pd.Series(values, index = intervals)
test = ser.rolling(window = offset, closed='left').min()
```