[WIP][FEATURE] implemented method to get all users in sulu system and tests

implemented method to get all users in sulu system and tests

__Tasks:__

- [ ] implement repository method
- [ ] test coverage
- [ ] gather feedback for my changes


__Informations:__

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| Tests pass?   | yes
| Fixed tickets | none
| Doc           | none