Function create_user 

pub async fn create_user(
    username: &str,
    email: &str,
    plaintext: &str,
) -> Result<AuthUser, AuthError>

Create a new active user with the given username, email, and plaintext password. The password is hashed before insert; the plaintext never touches the database. date_joined is set to Utc::now(); last_login is None; is_active = true, is_staff = false, is_superuser = false.