Backend support for first available vaccination slot

[LukasForst]

• we want to add possibility to display first available vaccination slot on the frontend
• the task here is to implement simple API (see VaccinationSlotRoutes.kt) that will return first available vaccination slot (if there's any) or 404 if there're no slots left (see NoVaccinationSlotsFoundException)

acceptation criteria:
There's existing endpoint with no authorization that allows user to obtain information about first available vaccination slot.
The route as well as the service is tested with unit test.