stream aggregation: add rate output

[hagen1778]

Is your feature request related to a problem? Please describe

Would be great to support rate output for stream aggregation as it is one of the most common functions used in recording rules or dashboards.

Describe the solution you'd like

To add rate output to stream aggregation which would calculate the per second change of the values for registered time series on the specified interval.

Describe alternatives you've considered

Alternative is to use increase output and additionally process the resulting time series by dividing it on the interval of aggregation to get per-second change.

[hagen1778]

cc @Amper