pam: introduce prompt_config struct
    
    prompt_config is the internal struct to control the prompting of
    pam_sss. To make it easy to change internal details when more options
    are added it should be opaque and only accessed by getters and setter.
    
    Related to https://pagure.io/SSSD/sssd/issue/3264
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>
    (cherry picked from commit fa8ef7c6db19a160d807f05b08bbc66c0c25ebfe)