Support OpenAI-compatible ChatMessage type

For language models, it'd be good to support an input type that's consistent with the OpenAI API.

I imagine something like this:

```python
from cog import Input
from cog.types import ChatMessage

def run(
    messages: list[ChatMessage] = Input(),
) -> str:
    print(messages)
    return f"HELLO {messages[0]['role']}"
```