[add-to-app/books] Added missing class declarations from the pigeon file. (#1385)

add_to_app/books/flutter_module_books/pigeon/schema.dart

@@ -2,9 +2,22 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-import 'package:flutter_module_books/api.dart';
 import 'package:pigeon/pigeon.dart';
 
+class Book {
+  String? title;
+  String? subtitle;
+  String? author;
+  String? summary;
+  String? publishDate;
+  int? pageCount;
+  Thumbnail? thumbnail;
+}
+
+class Thumbnail {
+  String? url;
+}
+
 @FlutterApi()
 abstract class FlutterBookApi {
   void displayBookDetails(Book book);