Add ExampleRepository.java and ExampleService.java to show how to make Spring service to interact with DB.

Added ServerHttpService.js, Constants.js, ExampleService.js, and changed HelloWorld.vue for show examples.
The front to back end connection is ready.

Change spring boot port to 8081, so it will not interoperate with Vue

"Added example controller and model. Fixed can not start app by comment the DB library, I will uncomment them when I need."