feat: OCC and OCS Calendar Import/Export

[SebastianKrupinski]

• Resolves: https://github.com/nextcloud-gmbh/customer-feature-requests/issues/563
• Requires: feat: OCC and OCS Calendar Import/Export calendar#6596

Summary

This adds the ability to export calendars via the OCS and OCC.

OCS

Endpoint: /ocs/v2.php/calendar/export
Request: GET/POST
{
"id": "personal",
"fmt": "xcal", (optional "ical, jcal, xcal", defaults to "ical")
"user": "user1" (optional admin permissions required)
}

OCC

Command: calendar:export
Arguments: userId calendarId format (optional) filepath

TODO

• Tests

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)