Add multi-user support for package installations across various modules

Updated multiple configuration files to include a `merge` option for user management, enhancing the ability to handle multi-user setups for applications and services. This change improves flexibility in managing user-specific package installations, ensuring a more streamlined configuration process.

parts/core.nix

@@ -237,6 +237,13 @@ in
           }) (inputs.self.lib.normalizeUsers users)
         );
       getFirstUser = users: if builtins.isString users then users else (builtins.head users);
+      mergeUsersOption =
+        lib: _loc: defs:
+        let
+          normalize = users: if builtins.isString users then [ users ] else users;
+          allUsers = lib.foldl' (acc: def: acc ++ (normalize def.value)) [ ] defs;
+        in
+        lib.unique allUsers;
     };
   };
 }