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/pubspec.yaml

@@ -0,0 +1,19 @@
+name: server
+description: A server app using the shelf package and Docker.
+version: 1.0.0
+publish_to: "none"
+
+environment:
+  sdk: ">=2.18.1 <3.0.0"
+
+dependencies:
+  args: ^2.0.0
+  shelf: ^1.1.0
+  shelf_router: ^1.0.0
+  shared:
+    path: ./shared
+
+dev_dependencies:
+  http: ^0.13.0
+  lints: ^2.0.0
+  test: ^1.15.0