volume scheduler: introduce special string type

This makes it possible to search for the special strings more easily
(https://github.com/kubernetes/kubernetes/pull/88230#discussion_r382367043).

pkg/scheduler/framework/plugins/volumebinding/volume_binding.go

@@ -81,7 +81,7 @@ func (pl *VolumeBinding) Filter(ctx context.Context, cs *framework.CycleState, p
 	if len(reasons) > 0 {
 		status := framework.NewStatus(framework.UnschedulableAndUnresolvable)
 		for _, reason := range reasons {
-			status.AppendReason(reason)
+			status.AppendReason(string(reason))
 		}
 		return status
 	}