Centralize logic for loading from LangChainHub, add ability to pin dependencies (#805)

It's generally considered to be a good practice to pin dependencies to
prevent surprise breakages when a new version of a dependency is
released. This commit adds the ability to pin dependencies when loading
from LangChainHub.

Centralizing this logic and using urllib fixes an issue identified by
some windows users highlighted in this video -
https://youtu.be/aJ6IQUh8MLQ?t=537

docs/modules/agents/examples/load_from_hub.ipynb

@@ -62,13 +62,26 @@
     "self_ask_with_search.run(\"What is the hometown of the reigning men's U.S. Open champion?\")"
    ]
   },
+  {
+   "attachments": {},
+   "cell_type": "markdown",
+   "id": "3aede965",
+   "metadata": {},
+   "source": [
+    "# Pinning Dependencies\n",
+    "\n",
+    "Specific versions of LangChainHub agents can be pinned with the `lc@<ref>://` syntax."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": null,
    "id": "e679f7b6",
    "metadata": {},
    "outputs": [],
-   "source": []
+   "source": [
+    "self_ask_with_search = initialize_agent(tools, llm, agent_path=\"lc@2826ef9e8acdf88465e1e5fc8a7bf59e0f9d0a85://agents/self-ask-with-search/agent.json\", verbose=True)"
+   ]
   }
  ],
  "metadata": {