[Pedometer] add Pedometer.stepStream

[LorenzSchueler]

Currently pedometer only provides one stream for changes in the pedestrian status and one for changes is the step count. This PR add another stream that yields every time a step is detected. It uses the step detection event channel which is already used by the pedestrian status stream but does not filter the events. I've tested in on Android and it works. Unfortunately I have no way to test it on ios, but as the event channel on ios seems to return the pedestrian status directly, I assume it will only yield on pedestrian status change and will therefor not work as expected.

[maratbekovaidar]

Nice