Expose federated calendars via caldav

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

I have a work Nextcloud and a private Nextcloud. In order to allow people on my private Nextcloud to see when I'm on a business trip I want to share the work calendar via federated sharing to some users on my private Nextcloud.

Sharing works great as long as you use the calendar in the browser. But most of the time people access their calendar via mobile devices these days and the federated calendar is not exposed via caldav at the Nextcloud of the recipient.

Describe the solution you'd like

Federated sharing should be completely transparent for the user. The files sync client also syncs shared files and folders shared via federation, the same way federated calendars should be exposed via CalDAV so that the user can also see the calendar on their mobile devices and desktop clients.

Describe alternatives you've considered

Do it like before we introduced federated calendar sharing: Create a public link and subscribe to it via the ICS link

Additional context

No response

No file or test is named. Start by tracing federated calendar sharing and the CalDAV exposure path in the Nextcloud Calendar app, then verify that a recipient's federated calendar is discoverable and usable through mobile and desktop CalDAV clients.