Implement resharing of incoming shared calendars

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

The CalDAV backend reports shared calendars as reshareable when they are shared with edit permissions. However, actually resharing the calendar does not work.

Describe the solution you'd like

It should be possible to reshare incoming shared calendars if the backend advertises it.

Describe alternatives you've considered

Do not advertise the resharing capability and drop this feature request.

Additional context

Response when trying to reshare an incoming shared calendar:

POST 403 https://localhost/remote.php/dav/calendars/admin/reshare-test-direct_shared_by_user/
<?xml version="1.0" encoding="utf-8"?>
<d:error xmlns:d="DAV:" xmlns:s="http://sabredav.org/ns">
  <s:exception>Sabre\DAV\Exception\Forbidden</s:exception>
  <s:message/>
</d:error>

However, the resharing capability is advertised for this calendar:

Potentially related code:
https://github.com/nextcloud/server/blob/aa5f037af71c915424c6dcfd5ad2dc82797dc0d6/apps/dav/lib/CalDAV/Publishing/PublishPlugin.php#L136-L139

Start with apps/dav/lib/CalDAV/Publishing/PublishPlugin.php around lines 136-139, then trace the CalDAV handling for a reshare POST on an incoming shared calendar. Reproduce the documented 403 response for an editable shared calendar and compare it with the advertised resharing capability. Done means an incoming shared calendar can be reshared when the backend advertises that capability.