Part of #19575

Currently, the system principal address book is not available from the AddressBookHome.

Todo

• Add the system address book while still respecting the share enumeration settings as defined in the Share API.
• Exclude any users coming from https://github.com/nextcloud/guests (uri: Guests:email.vcf)
• Exclude any disabled users (in my tests, they disappeared from the system address books).

Follow-ups

• Feature request: expose a guest users addressbook (use case still to be found)