add upcast to domain resource to cast to generic type

Author: jenschude

commercetools/commercetools-sdk-java-api/src/main/java/com/commercetools/api/models/DomainResource.java

@@ -4,4 +4,8 @@ public interface DomainResource<T> {
     public String getId();
 
     public Long getVersion();
+
+    default T upcast() {
+        return (T) this;
+    }
 }