add an option to disable syncing contact books to federated servers

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

my Nextcloud server federated with someone else's nextcloud server. as a result, their entire contact list has seemingly appeared on my own server without any prior notice. this is somewhat of a privacy breach especially with individual servers that are not hosted on some enterprise

Describe the solution you'd like

add an option in Administration settings to disable the automatic contact book syncing to federated instances and make it an opt-in by default.

Describe alternatives you've considered

at the moment, the only option you have is unfederating with any servers you have to stop the contact list from federating to the other servers.

Additional context

the 3rd most buton shows me the entire contact list of the other person. same for my own contact list on the other server.

attempting to view details for any of them leads to a 404 page

Start by locating the federation contact-book synchronization flow and the Administration settings entry point in Nextcloud Server. Determine how federated contact books are exposed and how an administrator-controlled opt-in or opt-out should apply. Done means administrators can disable the syncing and contact books no longer appear across federated servers when it is disabled.