provider yaml key is confusing

When I set up the tool with any provider but OpenAI, the keys seem to be confusing, for example

```bash
export OPENAI_API_KEY=sk-xxxxxxx

codegpt config set openai.provider gemini
```

`gemini` has nothing in common with `openai`. I understand that it's most likely a legacy, but it's better to switch this to `llm_config` and make it `llm_config.provider`.

A similar idea is about `OPENAI_API_KEY` env var. At this point, the env var for api_key is supported only by OpenAI, which is pretty inconvenient.