Create virtual address books from existing one by category

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

I have following problem / use case:
I don't want Whatsapp to check all my phone numbers against Whatsapp servers. Therefore I've created a separate address book for Whatsapp contacts only. This addressbook always gets out of sync with my main address book.

In business context I could create a virtual address book for each client but also have all contacts together in a main address book. I would have more control over address book shares: I only share contacts with persons that need the contact data.

Describe the solution you'd like

The user can create a virtual address book based on a specific category of an existing address book. Lets say for example:

• Category: "Foo"
• Existing address book: "Main"
• Virtual address book: "Virtual-Foo"

Only contacts with the category "Foo" appear in the "Virtual-Foo". When a new contact is pushed to "Virtual-Foo" it automatically gets the category "Foo". When accessing the a retrieving contact over the virtual address book, the category "Foo" is removed. This makes the process transparent to the client which uses "Virtual-Foo" (Imagine the user removes the category "Foo" manually on his client device this could cause problems which we can avoid here).

Main address book:

"Virtual-Foo" address book:

Describe alternatives you've considered

An alternative I see is to modify the client but this needs to be done for many platforms. A central solution would be better.

Additional context

No response

The issue names no files, tests, or entry points; start by locating the Contacts address-book and CardDAV handling, then trace how categories are stored and exposed. Done means a virtual address book filters by one category, synchronizes new contacts back to the source category, and hides that category when contacts are retrieved.