provider yaml key is confusing

[kvokka]

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

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.