CACHE_REQ: Add a new cache_req_ncache_filter_fn() plugin function
    
    This function will be responsible for filtering out all the results that
    we have that are also present in the negative cache.
    
    This is useful mainly for plugins which don't use name as an input token
    but can still be affected by filter_{users,groups} options.
    
    For now this new function is not being used anywhere.
    
    Related:
    https://pagure.io/SSSD/sssd/issue/3362
    
    Signed-off-by: Fabiano Fidêncio <fidencio@redhat.com>
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>