Allow setting up custom syncs

lib/modules/rclone.nix

@@ -12,6 +12,7 @@ let
   syncProviders = {
     nextcloud = mkProvider {};
     proton = mkProvider {};
+    manual = mkProvider {};
   };
 
   syncType = lib.types.submodule (
@@ -20,6 +21,7 @@ let
         type = lib.mkOption {
           description = "The type of the sync";
           type = lib.types.enum (builtins.attrNames syncProviders);
+          default = "manual";
         };
 
         autoStart = lib.mkOption {