Added possibility to chain includes. (#331)

* Added possibility to chain includes.

* Removed interface.

* Removed the need for generating GUIDs as ids and added tests.

tests/UnitTests/Builders/IncludeQueryBuilder.cs

@@ -0,0 +1,29 @@
+using Microsoft.eShopWeb.ApplicationCore.Helpers.Query;
+using Microsoft.eShopWeb.ApplicationCore.Interfaces;
+using Microsoft.eShopWeb.UnitTests.ApplicationCore.Helpers.Query;
+using System;
+using System.Collections.Generic;
+
+namespace Microsoft.eShopWeb.UnitTests.Builders
+{
+    public class IncludeQueryBuilder
+    {
+        public IncludeQuery<Person, List<Person>> WithCollectionAsPreviousProperty()
+        {
+            var pathMap = new Dictionary<IIncludeQuery, string>();
+            var query = new IncludeQuery<Person, List<Person>>(pathMap);
+            pathMap[query] = nameof(Person.Friends);
+
+            return query;
+        }
+
+        public IncludeQuery<Book, Person> WithObjectAsPreviousProperty()
+        {
+            var pathMap = new Dictionary<IIncludeQuery, string>();
+            var query = new IncludeQuery<Book, Person>(pathMap);
+            pathMap[query] = nameof(Book.Author);
+
+            return query;
+        }
+    }
+}