Update notification-controller to new unified standards

[scottrigby]

• Part of Roll out new unified standards to all Flux controllers flux2#1601

This issue is to track rolling out the changes to Notification Controller.

This controller is sufficiently complex to make exemplary use of pkg/runtime, specifically it's primary purpose which is move conditions logic to a reusable package. Once this is updated, it should help steer the way for a wider group to roll these standards out to the other controllers.

[stefanprodan]

@scottrigby please open these in each controller repo, I moved the helm one.