Add moar reserved keywords to validation list (#734)

Add more reserved keywords to validation list. These keywords are
reserved to support potential future functionality where users may
need to access the root node or other graph primitives.

Author: a-hilaly

pkg/graph/validation.go

@@ -55,7 +55,9 @@ var (
 		"object",
 		"resource",
 		"resourcegraphdefinition",
+		"resourceGraphDefinition",
 		"resources",
+		"root",
 		"runtime",
 		"schema",
 		"self",