Add option to import credentials from environment variables for private Sheets

[LeoArtaza]

I was using this for a project and it works well when deploying on Streamlit Community Cloud, but I wanted to deploy on another service to have a custom domain. However, other services don't look for streamlit secrets file specifically but rather custom environment variables, and as far as I can tell, there is no way of establishing a connection with a private Sheets file without a streamlits secrets file specifically.