initial upload of extension sample

srv/extension_service.cds

@@ -0,0 +1,9 @@
+using from '_base/srv/admin-service';
+using from '_base/srv/cat-service';
+using from '_base/srv/orders-service';
+
+using { sap.bookshop.extension as ext } from '../db/extension';
+
+extend service OrdersService with {
+   entity Customers as projection on ext.Customers;
+}