nixos/webdav: add package option

nixos/modules/services/network-filesystems/webdav.nix

···
     services.webdav = {
       enable = lib.mkEnableOption "WebDAV server";
 
+      package = lib.mkPackageOption pkgs "webdav" { };
+
       user = lib.mkOption {
         type = lib.types.str;
         default = "webdav";
···
       after = [ "network.target" ];
       wantedBy = [ "multi-user.target" ];
       serviceConfig = {
-        ExecStart = "${pkgs.webdav}/bin/webdav -c ${cfg.configFile}";
+        ExecStart = "${lib.getExe cfg.package} -c ${cfg.configFile}";
         Restart = "on-failure";
         User = cfg.user;
         Group = cfg.group;