Add more flexible ways of adding time ranges to appointments

Is your feature request related to a problem? Please describe.

I am running a nextcloud at university. The Appointment feature could be very handy for most of our professors: Students could book professors’ consultation-hours. There are different sorts of appointment dates:

• Many professors offer weekly consultation-hours while the semester and some loose dates within the semester-breaks.
• Visiting professors only offer loose dates.
• Some professors have different days for consultation-hours in winter-semester and in summer-semester.
• While a professor has a sabbatical term there are no consultation-hours offered.

Describe the solution you'd like

I would like to have the following options:

• offer time ranges for a given number of times
• offer time ranges until a given date
• save multiple sets of time ranges which can be acitvated/deactivated by date
• ability to set loose dates only or in addition to defined time ranges
• ability to define time ranges in other than weekly recurrences

Describe alternatives you've considered

Use https://www.srgdev.com/lab/nextcloud-appointments/ which brings other disadvantages.

Additional context

No response

No files or tests are named. Start by locating the existing Appointment time-range and recurrence entry points, then compare their behavior with each requested scheduling mode; done means the supported combinations and activation rules are defined and covered by appropriate tests.