refactor timeslot API filters

Konrad Mohrfeldtrequested to merge
kmohrf/timeslot-filters into main

This MR:

  • removes default values for start and end filter (see #189 (closed))
  • renames start to startsAfter and end to endsBefore
  • adds datetime support to startsAfter and endsBefore
  • adds new datetime filter startsBefore and endsAfter

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