add read only tree table and value help

bookshop/srv/cat-service.cds

@@ -1,4 +1,7 @@
 using { sap.capire.bookshop as my } from '../db/schema';
+using {my.common.Hierarchy as Hierarchy} from './hierarchy';
+
+extend my.Genres with Hierarchy;
 service CatalogService @(path:'/browse') {
 
   /** For displaying lists of Books */
@@ -10,6 +13,9 @@ service CatalogService @(path:'/browse') {
     author.name as author
   } excluding { createdBy, modifiedBy };
 
+  @readonly
+  entity GenreHierarchy as projection on my.Genres;
+
   @requires: 'authenticated-user'
   action submitOrder ( book: Books:ID, quantity: Integer ) returns { stock: Integer };
   event OrderedBook : { book: Books:ID; quantity: Integer; buyer: String };