PreEnqueue implementation

- Add PreEnqueuePlugin to Scheduler Framework
- Implement PreEnqueuePlugin in scheduler queue
- Implementation of SchedulingGates plugin
- Metrics

pkg/scheduler/framework/plugins/feature/feature.go

@@ -25,4 +25,5 @@ type Features struct {
 	EnableMinDomainsInPodTopologySpread          bool
 	EnableNodeInclusionPolicyInPodTopologySpread bool
 	EnableMatchLabelKeysInPodTopologySpread      bool
+	EnablePodSchedulingReadiness                 bool
 }