Add game_template (#1180)

Adds a template / sample for games built in Flutter, with all the bells and whistles, like ads, in-app purchases, audio, main menu, settings, and so on.

Co-authored-by: Parker Lougheed
Co-authored-by: Shams Zakhour

game_template/lib/firebase_options.dart

@@ -0,0 +1,12 @@
+// File normally generated by FlutterFire CLI. This is a stand-in.
+// See README.md for details.
+import 'package:firebase_core/firebase_core.dart' show FirebaseOptions;
+
+class DefaultFirebaseOptions {
+  @Deprecated('Run `flutterfire configure` to re-generate this file')
+  static FirebaseOptions get currentPlatform {
+    throw UnimplementedError(
+        'Generate this file by running `flutterfire configure`. '
+        'See README.md for details.');
+  }
+}