Support in-process priorirty plugins

[ordovicia]

type priorities.PriorityConfig struct {
    priorities.PriorityMapFunction,
    priorities.PriorityReduceFunction,
    ...
}

type priorities.PriorityMapFunction
  = func(pod *v1.Pod, meta interface{}, nodeInfo *nodeinfo.NodeInfo) (api.HostPriority, error)
type priorities.PriorityReduceFunction
  = func(pod *v1.Pod, meta interface{}, nodeNameToInfo map[string]*nodeinfo.NodeInfo, result api.HostPriorityList) error