Provide an option to write the openai api key in config directly instead of reading it from os env

[LorNtz]

My case might be rare, but I recently installed this plugin on multiple machines with different os and it annoys me when I have to set env variable on different os in different ways. Updating the key on each of them could also be a problem for me. I think it will be great to provide an option in config to write the api key string directly so that it can be carried across different environments with my neovim config.