Add support Vertex AI Gemini uses a public image URL (#14949)

## What

Since `langchain_google_genai.ChatGoogleGenerativeAI` supported A public
image URL, we add to support it in `langchain.chat_models.ChatVertexAI`
as well.

### Example

```py
from langchain.chat_models.vertexai import ChatVertexAI
from langchain_core.messages import HumanMessage

llm = ChatVertexAI(model_name="gemini-pro-vision")
image_message = {
    "type": "image_url",
    "image_url": {
        "url": "https://python.langchain.com/assets/images/cell-18-output-1-0c7fb8b94ff032d51bfe1880d8370104.png",
    },
}
text_message = {
    "type": "text",
    "text": "What is shown in this image?",
}
message = HumanMessage(content=[text_message, image_message])

output = llm([message])
print(output.content)
```

## Refs

-
https://python.langchain.com/docs/integrations/llms/google_vertex_ai_palm
-
https://python.langchain.com/docs/integrations/chat/google_generative_ai

docs/docs/integrations/llms/google_vertex_ai_palm.ipynb

@@ -539,6 +539,35 @@
     "print(output2.content)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "You can also use the public image URL:"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "image_message = {\n",
+    "    \"type\": \"image_url\",\n",
+    "    \"image_url\": {\n",
+    "        \"url\": \"https://python.langchain.com/assets/images/cell-18-output-1-0c7fb8b94ff032d51bfe1880d8370104.png\",\n",
+    "    },\n",
+    "}\n",
+    "text_message = {\n",
+    "    \"type\": \"text\",\n",
+    "    \"text\": \"What is shown in this image?\",\n",
+    "}\n",
+    "message = HumanMessage(content=[text_message, image_message])\n",
+    "\n",
+    "output = llm([message])\n",
+    "print(output.content)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {