add minimalistic rigths grouping system by extending the yaml with aditional Key "group"

[sgaechter]

In one of my Streamlit applications, I wanted to distribute permissions to different users using groups. Therefore I extended Streamlit-Authenticator with another field. For this, I created the key "users" in YAML. So that existing cookies do not lead to a key error I extended the function " _token_decode" by a query, which sets the "st.session_state['group']" to "None" if the object is not present.