Federation: Sync user groups with `federation:sync-addressbooks`

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

Currently, federation allows to synchronize the address books of two servers, allowing to use auto completion when sharing to users on the other server. However, this does not apply to user groups.

While it is already possible to share with a remote group by typing the federated Cloud-ID and then selecting "Group on server xyz", there is no auto completion for this feature. Since the federated ID does not always correspond to the actual name of the group (e.g. when the SAML user backend is used, groups have a special prefix), it is sometimes hard to find the correct ID.

Describe the solution you'd like

(Possibly optionally) Allow to synchronize remote groups together with the rest of the address book for trusted servers. Add auto completion for remote groups in the sharing menu.

Describe alternatives you've considered

Additional context

Start by tracing the federation:sync-addressbooks command and the sharing menu flow. Determine how trusted-server address books currently represent users, then define the scope for remote groups and their autocomplete entries. Done means the agreed behavior is implemented and remote groups can be found and shared through the menu.