`gemini` completion doesn't handle message contents as a list

[daavoo]

OpenAI API supports the following format (https://platform.openai.com/docs/api-reference/chat/create#chat-create-messages-user-message-content):

messages=[
    {
        "role": "user", 
        "content":  {
            "type": "text",
            "text": "HELLO!"
        }
    }
]

But our current gemini implementation doesn't handle it correctly and fails at:

any-llm/src/any_llm/providers/gemini/utils.py

Line 81 in a97ec90

parts = [types.Part.from_text(text=message["content"])]

Apart from being a bug today for text type, this a prerequisite for #415