Sync availability

[dmolineus]

Some ics calendars uses a vendor header such like X-MICROSOFT-CDO-BUSYSTATUS from MS teams calendars to provide availability information. Even though this is not part of the official ics standard, it would be great if such information are synced as well. Android seems to support availability, see
https://developer.android.com/reference/android/provider/CalendarContract.EventsColumns#AVAILABILITY