Web Loader: Add proxy support (#6792)

Proxies are helpful, especially when you start querying against more
anti-bot websites.

[Proxy
services](https://developers.oxylabs.io/advanced-proxy-solutions/web-unblocker/making-requests)
(of which there are many) and `requests` make it easy to rotate IPs to
prevent banning by just passing along a simple dict to `requests`.

CC @rlancemartin, @eyurtsev

docs/extras/modules/data_connection/document_loaders/integrations/web_base.ipynb

@@ -224,13 +224,33 @@
     "docs"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "source": [
+    "## Using proxies\n",
+    "\n",
+    "Sometimes you might need to use proxies to get around IP blocks. You can pass in a dictionary of proxies to the loader (and `requests` underneath) to use them."
+   ],
+   "metadata": {
+    "collapsed": false
+   }
+  },
   {
    "cell_type": "code",
    "execution_count": null,
-   "id": "1dd8ab23",
-   "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "loader = WebBaseLoader(\n",
+    "    \"https://www.walmart.com/search?q=parrots\", proxies={\n",
+    "        \"http\": \"http://{username}:{password}:@proxy.service.com:6666/\",\n",
+    "        \"https\": \"https://{username}:{password}:@proxy.service.com:6666/\"\n",
+    "    }\n",
+    ")\n",
+    "docs = loader.load()\n"
+   ],
+   "metadata": {
+    "collapsed": false
+   }
   }
  ],
  "metadata": {