Improve checking process for booked appointments

[adewes]

Currently the user app regularly fetches all available appointments to verify whether the booked appointment is still valid. We should improve this process as in the future the app might not always be able to retrieve all appointments from the backend.

Specifically, the app currently checks that the booked slot is still listed in the appointments published by the API. As the number of providers grows large for a given zip code area we might not always return all available appointments via the API endpoint, but rather a small but sufficient selection.

Hence, in order to check that the appointment is still advertised we should add an endpoint that either confirms this for a given slot ID (which is not privacy-friendly though) or an endpoint that retrieves all appointments by a given provider (also not privacy-friendly but slightly better).

We could also try to modify the getAppointmentsByZipCode endpoint such that it always returns a stable set of providers for a given user, this could also undermine the privacy of the user though. In general, the system currently assumes that it's possible to return all relevant appointments in a given zip code area to the user.