dav: Move displayname to common properties

crates/carddav/src/address_object/resource.rs

@@ -64,6 +64,10 @@ impl Resource for AddressObjectResource {
         })
     }
 
+    fn get_displayname(&self) -> Option<&str> {
+        self.object.get_full_name()
+    }
+
     fn get_owner(&self) -> Option<&str> {
         Some(&self.principal)
     }