sd-id128: export sd_id128_get_app_specific()

sd-id128: export sd_id128_get_app_specific()

We exported sd_id128_get_machine_app_specific() and
sd_id128_get_boot_app_specific(). But those operate on the current machine,
and sometimes the user might want to do the same calculation for a different
machine, or an earlier boot, etc. We expose the underlying implementation
to make this easy.