[useChat] Support for changing the system prompt

[s-h-a-d-o-w]

I don't see a way how one could do this currently, since there's no built-in way of doing it and also no way of passing additional data to the API.

[lgrammel]

You would set the system prompt on the server, e.g. in streamText. If you really really want to set the system prompt client-side, you can manually add a message with system role first.

[s-h-a-d-o-w]

Yes, I absolutely want GPT's personality to be customizable. Good to know that this is the way to do it (without a round trip to some other API endpoint anyway), thanks!