feat: Introduced method `createUser()` in `UserService`

feat: Introduced method `createUser()` in `UserService`

* The method requests the creation of a new user and remembers the created
  instance of `User`, if the requests ist successful.
* The remembered `User` can be retrieved via `UserService.getUser()`.
* Also added a pact between the `UserService` and the
  `ChatBackendController`, that formulates the expectations of the
  `UserService`, that are not implemented yet by the backend.