Workload-aware preemption

This KEP describes the changes to kube-scheduler to support workload-aware preemption. We focus on the API, framework and building blocks, not the ideal algorithm - it can come as a follow up. We start with simple implementation, that is heavily based on the existing pod preemption algorithm. The Workload and PodGroup API introduced in [KEP-4671: Gang Scheduling using Workload Object] is extended to allow expressing the concept of pod group priority and to define the preemption unit. With those extensions we make the next step towards our workload-aware scheduling north star.