Better support for xAI/grok for AI configuration

[jovezhong]

Description

x.ai's grok3 is fast(and cheap). Better to have native support in marimo

Suggested solution

• Add 4th item in 'AI KEYS'
• in the AI ASSIST section, check the model name, if it starts from grok, then use the grok API, not OpenAI API Key

I need to use both OpenAI and Grok, so I don't want to change the OpenAI key to the grok, but in latest version, it will call api.x.ai, with grok3 model but with openai key, ending up with error

Alternative

No response

Additional context

No response