Allow to specify a custom loader for GcsFileLoader (#8868)

Co-authored-by: Leonid Kuligin <kuligin@google.com>

docs/extras/integrations/document_loaders/google_cloud_storage_file.ipynb

@@ -73,13 +73,27 @@
     "loader.load()"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "id": "41c8a46f",
+   "metadata": {},
+   "source": [
+    "If you want to use an alternative loader, you can provide a custom function, for example:"
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
    "id": "eba3002d",
    "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "from langchain.document_loaders import PyPDFLoader\n",
+    "def load_pdf(file_path):\n",
+    "    return PyPDFLoader(file_path)\n",
+    "\n",
+    "loader = GCSFileLoader(project_name=\"aist\", bucket=\"testing-hwc\", blob=\"fake.pdf\", loader_func=load_pdf)"
+   ]
   }
  ],
  "metadata": {