Add misc preferences

lib/core/prefs/misc_prefs.dart

@@ -0,0 +1,10 @@
+import 'package:hiddify/domain/constants.dart';
+import 'package:hiddify/utils/pref_notifier.dart';
+
+final connectionTestUrlProvider =
+    PrefNotifier.provider("connection_test_url", Defaults.connectionTestUrl);
+
+final concurrentTestCountProvider = PrefNotifier.provider(
+  "concurrent_test_count",
+  Defaults.concurrentTestCount,
+);