nixos/syncthing: fix path setting for versioning

Syncthing config XML uses `fsPath` setting for specifying the path to the versioning folder. This commit adds `services.syncthing.folders.<name>.versioning.fsPath` option to enable this functionality declaratively. Previously, `versioning.params.versionsPath` was used, which doesn't work.

nixos/modules/services/networking/syncthing.nix

@@ -268,10 +268,10 @@ in {
                   {
                     versioning = {
                       type = "staggered";
+                      fsPath = "/syncthing/backup";
                       params = {
                         cleanInterval = "3600";
                         maxAge = "31536000";
-                        versionsPath = "/syncthing/backup";
                       };
                     };
                   }
@@ -296,6 +296,14 @@ in {
                       See <https://docs.syncthing.net/users/versioning.html>.
                     '';
                   };
+                  fsPath = mkOption {
+                    default = "";
+                    type = either str path;
+                    description = mdDoc ''
+                      Path to the versioning folder.
+                      See <https://docs.syncthing.net/users/versioning.html>.
+                    '';
+                  };
                   params = mkOption {
                     type = attrsOf (either str path);
                     description = mdDoc ''