[YUNIKORN-810]Add a field in scheduler-interface to represent required node for the scheduler (#46)

Add requiredNode field to interface.
diff --git a/lib/go/common/constants.go b/lib/go/common/constants.go
index 6eb21e8..561d32e 100644
--- a/lib/go/common/constants.go
+++ b/lib/go/common/constants.go
@@ -48,6 +48,7 @@
 	GroupAnnotation = "annotation/"
 
 	// Keys
-	KeyPodName   = "podName"
-	KeyNamespace = "namespace"
+	KeyPodName      = "podName"
+	KeyNamespace    = "namespace"
+	KeyRequiredNode = "requiredNode"
 )