feat: introduce strict eviction policy

with strict eviction policy the descheduler only evict pods if the pod
contains a request for the given threshold. for example, if using a
threshold for an extended resource called `example.com/gpu` only pods
who request such a resource will be evicted.

pkg/framework/plugins/nodeutilization/validation.go

@@ -30,7 +30,25 @@ func ValidateHighNodeUtilizationArgs(obj runtime.Object) error {
 	if err != nil {
 		return err
 	}
+	// make sure we know about the eviction modes defined by the user.
+	return validateEvictionModes(args.EvictionModes)
+}
 
+// validateEvictionModes checks if the eviction modes are valid/known
+// to the descheduler.
+func validateEvictionModes(modes []EvictionMode) error {
+	// we are using this approach to make the code more extensible
+	// in the future.
+	validModes := map[EvictionMode]bool{
+		EvictionModeOnlyThresholdingResources: true,
+	}
+
+	for _, mode := range modes {
+		if validModes[mode] {
+			continue
+		}
+		return fmt.Errorf("invalid eviction mode %s", mode)
+	}
 	return nil
 }