Provide appointment slots for a given time span via REST API

[ChristophWurst]

• Extrapolate slots based on availability, duration, increment, …
• Exclude days where the max number of slots is reached
• Exclude slots that are not available a booking conflict
  • Requires Add an API for calendar providers server#28970
  • Option to set a calendar as transparent for free/busy #3193 would possibly be a good addition to the logic. We need to get access to calendar properties to filter out the transparent ones. This is possibly a task for later on. If so, let's create a separate ticket so we don't forget.