Support ChatGPT Pro/Plus login

**What you'd like to see**

Currently the OpenAI provider requires an API key.

It would be great to also support authenticating with a ChatGPT Plus/Pro subscription via OAuth, so users who already pay for ChatGPT can use their included model access without a separate API billing account.

Not sure if OpenAI allows this, but https://opencode.ai/docs/providers#openai supports this