CalDAV/Calendar export and import for user migration

[ChristophWurst]

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

As a Nextcloud server admin I …

Describe the solution you'd like

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Describe alternatives you've considered

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Additional context

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[SebastianKrupinski]

Related: #55178

[ChristophWurst]

This task will be split:

1. Export/import calendars+events (MVP, medium priority)
2. Export/import appointment configs (follow-up, low priority)
3. Export/import meeting proposals (follow-up, low priority)

@SebastianKrupinski split this accordingly and bring this ticket into shape. It should look like the structure documented at https://github.com/nextcloud/groupware/blob/main/resources/task_ticket_structure.md so that we can list the work packages and follow-ups. Use nextcloud/mail#12001 as example.

[SebastianKrupinski]

Overview Ticket

nextcloud/calendar#7793

Calendar Migrator

nextcloud/calendar#7792

DAV Migrator Improvements

#57112