Add code sharing sample (#1444)

* code-sharing boilerplate

* initial commit of code-sharing logic

* documentation improvements

* added code-sharing to samples.yaml

* Updated counter UI to visually indicate communication with the server

* added code_sharing to CI

* CI changes to code_sharing

* fixed test with DI

* formatting

* added shared module to CI

* adds forgotten CI change

code_sharing/client/test/widget_test.dart

@@ -0,0 +1,15 @@
+import 'package:flutter_test/flutter_test.dart';
+import 'package:client/main.dart';
+
+void main() {
+  group('App should', () {
+    testWidgets('build its widgets', (WidgetTester tester) async {
+      await tester.pumpWidget(
+        MyApp(
+          getCount: () => Future.value(1),
+          increment: (int x) => Future.value(x),
+        ),
+      );
+    });
+  });
+}