Make it possible to load api keys from a separate file

[einarpersson]

Is your feature request related to a problem? Please describe.
I want to put my aichat/config.yaml in my dotfiles git repo, and hence I want to get rid of the api keys from that particular file. I saw that there is an alternative which is to specify the keys as environment variables OPENAI_API_KEY CLAUDE_API_KEY etc. But sourcing (eg. in .zshenv) will make them visible to all subprocesses and be outputted in any dump of env variables, making them prone to leaking as of my understanding. Also the naming is very general, I will probably use OpenAi api keys in other contexts.

Describe the solution you'd like
Perhaps a AICHAT_API_KEYS_FILE env variable or something like that, with a sensible default (sibling to the config.yaml file), so that the application itself can read from the file directly.

If not, at least the names of the api keys should be prefixed with AICHAT, shouldn't them?

Additional context
It may be that I lack understanding on this topic, I just want to do things the right way :)