feat: allow to pass SlotSearchType to find-appointment command

[dawidgora]

Changes

• Add SlotSearchType support to find-appointment, including a --slot-search-type/-S flag and propagation
  through AppointmentFinder.
• Introduce DEFAULT_SLOT_SEARCH_TYPE and a parser helper so the API payload can accept either numeric or
  string slot types.
• Update unit tests to assert the new parameter is sent to the API and that the CLI wiring handles both
  explicit values and the default.

[dawidgora]

FYI @rafsaf

[rafsaf]

Thanks a lot, looks fine at first glance but it will need another look today or tomorrow