Well, I actually hoped that there was a feature like this too but as far as I know, there isn't as at the moment.
What can be done though is ask a user to choose his/her role while registering/signing up, and then this role gets stored with the user's other details that may be needed; then when this user wants to login, your application checks if the user is a registered user and then checks the database for the user's details (the role inclusive) and then the app works with the user based on this role.
Not a very good implementation but the closest I can proffer because this is the method I use
You can check out this question too how do I implement role based access control in firebase