feat(account): add event for fetching more notifications

Add a new event InAppNotificationCenterFetchMoreRequested to be dispatched
when the user scrolls to the end of a notification list and more data needs
to be fetched.

Author: fulleni

lib/account/bloc/in_app_notification_center_event.dart

@@ -55,3 +55,10 @@ class InAppNotificationCenterMarkOneAsRead
   @override
   List<Object> get props => [notificationId];
 }
+
+/// Dispatched when the user scrolls to the end of a notification list and
+/// more data needs to be fetched.
+class InAppNotificationCenterFetchMoreRequested
+    extends InAppNotificationCenterEvent {
+  const InAppNotificationCenterFetchMoreRequested();
+}