Add support for a shell command in place of environment variable name

[abayomi185]

Having the API key in environment variables is not always safe, as any application or program can read it.

I also had a situation where a program didn't load the project specific .env file but used the main key from the user profile - although, this case can be rectified with the recent pattern using a different variable name.

This PR allows for this:

chat = require("codecompanion.adapters").use("openai", {
  env = {
    api_key = "cmd:gpg --decrypt ~/.openai-api-key.gpg 2>/dev/null",
  },
}),

I'm happy to make further changes; this change breaks assumptions about what the config should be.

[olimorris]

Thank you and great suggestion.

More than happy to add this to the plugin. Could you update the readme to reflect this too? I think this would be a popular addition.

[abayomi185]

Yes. README has been updated.

[olimorris]

Fantastic and thanks again. I will make sure I use this for my own API keys