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>