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/server/shared/lib/shared.dart

@@ -0,0 +1,4 @@
+/// Common data models required by our client and server.
+library shared;
+
+export 'src/models.dart';