Default Pod Topology Spread

With Pod Topology Spread, workload authors can define spreading rules for their loads based on the topology of the clusters. The spreading rules are defined in the PodSpec, thus they are tied to the pod. We propose the introduction of configurable default spreading constraints, i.e. constraints that can be defined at the cluster level and are applied to pods that don't explicitly define spreading constraints. This way, all pods can be spread according to (likely better informed) constraints set by a cluster operator. Workload authors don't need to know the topology of the cluster they will be running on to have their pods spread. But if they do, they can still set their own spreading constraints if they have specific needs.