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/Makefile

@@ -0,0 +1,24 @@
+# MacOS/Linux-only. Sadly, this is only helpful as documentation for Windows.
+
+# LOCAL / DOCKER
+run-server:
+	# Depends on `make build`
+	docker-compose up -d
+
+stop-server:
+	docker-compose stop
+
+# Flutter
+flutter-local:
+	cd client && flutter run -d macos
+
+test:
+	(cd shared && dart test)
+	(cd server && dart test)
+	(cd client && flutter test)
+
+# Docker
+build:
+	(cd server && flutter pub get)
+	docker build ./server -t codesharingserver -f server/Dockerfile
+