Convert from being a Custom Pod Autoscaler to being controller based

[jthomperoo]

Is your feature request related to a problem? Please describe.
The Custom Pod Autoscaler approach used in this project has been really useful for getting a working version ready and out the door, but now it would be better to mature this project by converting this to being an operator focused autoscaler.

This would mean that the PHPA would have its own CRD, which would be managed by a single controller which handles all of the scaling logic for every PHPA.