[API, Feature] Support response prefill for openai API

[Ying1123]

Support response prefill for openai API like what anthropic supports:
https://docs.anthropic.com/en/docs/build-with-claude/prompt-engineering/prefill-claudes-response

Example usage:

# Launch server
python -m sglang.launch_server --model-path meta-llama/Llama-2-7b-chat-hf --port 30000

# Send request
import openai
from openai import OpenAI


client = openai.Client(base_url="http://127.0.0.1:30000/v1", api_key="EMPTY")

response = client.chat.completions.create(
    model="meta-llama/Meta-Llama-3.1-8B-Instruct",
    messages=[
        {"role": "system", "content": "You are a helpful AI assistant"},
        {
            "role": "user",
            "content":
"""
Extract the name, size, price, and color from this product description as a JSON object:

<description>
The SmartHome Mini is a compact smart home assistant available in black or white for only $49.99. At just 5 inches wide, it lets you control lights, thermostats, and other connected devices via voice or app—no matter where you place it in your home. This affordable little hub brings convenient hands-free control to your smart devices.
</description>
""",
        },
        {
            "role": "assistant",
            "content": "{\n",
        },
    ],
    temperature=0,
)

print(response.choices[0].message.content)

Output:

"name": "SmartHome Mini",
"size": "5 inches wide",
"price": "$49.99",
"color": "black or white"
}

Without the prefilled response of "{\n", the output is:

Here's the extracted information as a JSON object:

```json
{
"name": "SmartHome Mini",
"size": "5 inches wide",
"price": "$49.99",
"color": "black or white"
}
```

If you want to store the color as an array of strings, you could modify the JSON object like this:
```json
{
"name": "SmartHome Mini",
"size": "5 inches wide",
"price": "$49.99",
"color": ["black", "white"]
}
```