Appointments availability

[ChristophWurst]

Due some technical barriers we haven not added availability configuration to appointments.

Users should be able to pick between

• Default (scheduling inbox) availability
• Custom availability

This needs

• An appropriate data structure as defined in https://github.com/nextcloud/calendar-availability-vue Document target data structure calendar-availability-vue#7
• A UI (similar to Add scheduling availability settings server#27466)
• Use working hours as appointment availability default #3694
• Backend to get and set the availability - see Add the Availability Logic to slot generation #3689
• Serialization into the db record

[ChristophWurst]

first version of the new component (lib) https://github.com/nextcloud/calendar-availability-vue