(First task): add GET all users endpoint

## Problem
Add route for API, that would be available by `GET /user` endpoint
it should return all users from the database

## Steps
- Add `GET /` endpoint for user router
- Add `getAllUsers()` method for `UserService`
- Add `getAllUsers()` method for `UserRepository` 
- Add `getAllUsers()` method for `UserSettings`
- Add tests for the route