Shift SDL extension interface implementation

Author: dzhou-atlassian

src/main/java/graphql/language/InterfaceTypeExtensionDefinition.java

@@ -14,7 +14,7 @@
 import static graphql.collect.ImmutableKit.emptyList;
 
 @PublicApi
-public class InterfaceTypeExtensionDefinition extends InterfaceTypeDefinition {
+public class InterfaceTypeExtensionDefinition extends InterfaceTypeDefinition implements SDLExtensionDefinition {
 
     @Internal
     protected InterfaceTypeExtensionDefinition(String name,