Support for multi content chat message

[Neofox]

Hello, I noticed while trying to use the library that ChatMessage only support content as a string.

example:

chat_response = client.chat.complete(
    model = model,
    messages = [
        {
            "role": "user",
            "content": "What is the best French cheese?",
        },
    ]
)

but the actual mistral API like most other LLM support multi content messages, which is extremely useful when trying to send an image as part of a message.
Link to the official documentation: https://docs.mistral.ai/capabilities/vision/

as you can see, a common use case would be to send a message with multipart content.
example:

messages = [
    {
        "role": "user",
        "content": [
            {
                "type": "text",
                "text": "What's in this image?"
            },
            {
                "type": "image_url",
                "image_url": "https://tripfixers.com/wp-content/uploads/2019/11/eiffel-tower-with-snow.jpeg"
            }
        ]
    }
]

So if we could add the support to this feature it would be pretty nice.
I will start working on the PR, as I don't think it will be complicated to add as a feature.