Enable OpenAI compatible endpoints

I'm curious if there would be interest in accepting a PR that enables OpenAI endpoint (e.g., vllm) in addition to Gemini. I don't want to send the review without knowing if that's something this project would support or want.

```
// AX_PLANNER_TYPE - "gemini" or "openai"
// AX_OPENAI_BASE_URL - OpenAI-compatible endpoint
// AX_OPENAI_MODEL - model name
// AX_OPENAI_TIMEOUT - request timeout (e.g. "60s")
// AX_OPENAI_API_KEY_ENV - name of the env var holding the API key
```

I have it implemented in a local fork, but have not tested all of the demos to verify all of the functionality is there yet.