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/pubspec.yaml

@@ -0,0 +1,17 @@
+name: shared
+description: Common data models required by our client and server
+version: 1.0.0
+
+environment:
+  sdk: ">=2.18.1 <3.0.0"
+
+dependencies:
+  freezed_annotation: ^2.1.0
+  json_annotation: ^4.7.0
+
+dev_dependencies:
+  build_runner: ^2.2.1
+  freezed: ^2.1.1
+  json_serializable: ^6.4.0
+  lints: ^2.0.0
+  test: ^1.16.0