feat(customization): new customization service api (#4688)

extensions/default/src/customizations/helloPageCustomization.tsx

@@ -0,0 +1,14 @@
+import React from 'react';
+
+export default {
+  'routes.customRoutes': {
+    routes: {
+      $push: [
+        {
+          path: '/custom',
+          children: () => <h1 style={{ color: 'white' }}>Hello Custom Route</h1>,
+        },
+      ],
+    },
+  },
+};