KCM; Hide the secret URL as implementation detail instead of exposing it in the JSON-marshalling API
    
    There were several functions in the kcmsrv_ccache_json.c module that
    were really specific to storing ccaches in the secrets responder. Since
    we are about to introduce a new ccache back end which will on one hand
    use the JSON representation but on the other hand not use the secrets
    responder, we move several functions and definitions to
    kcmsrv_ccache_secrets.c
    
    A function to create the key for the secret is on the other made public
    so that it could be reused in the future.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3685
    
    Reviewed-by: Fabiano Fidêncio <fidencio@redhat.com>