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/pubspec.yaml

@@ -0,0 +1,23 @@
+name: client
+description: A Flutter app which communicates with a Dart backend using shared business logic.
+publish_to: "none"
+version: 1.0.0+1
+
+environment:
+  sdk: ">=2.18.1 <3.0.0"
+
+dependencies:
+  cupertino_icons: ^1.0.2
+  flutter:
+    sdk: flutter
+  http: ^0.13.5
+  shared:
+    path: ../server/shared
+
+dev_dependencies:
+  flutter_lints: ^2.0.0
+  flutter_test:
+    sdk: flutter
+
+flutter:
+  uses-material-design: true