Add base class for calendar plugins
This allows applications/services (e.g. Akonadi, Sink, KHolidays etc) to provide calendar content to other applications/services (e.g. KOrganizer, Plasma, Calindori).
The plugins expose a set of Calendars.
This is intended to replace the calendar plugin system found in https://invent.kde.org/frameworks/kdeclarative/-/tree/master/src/calendarevents