openai-compatible model: Allow system prompt

[DoS007]

trafficstars

Feature Request

The openai-compatible model implementation is really nice: https://github.com/nomic-ai/gpt4all/pull/2683#issue-2413048592 .

It would be good if for the model a system prompt would be configurable (sent as an invisible first message on every new chat). That way the type of chat is configurable to suit one needs.