sd128: export sd_id128_to_uuid_string()

sd128: export sd_id128_to_uuid_string()

We expose various other forms of UUID helpers already, i.e.
SD_ID128_UUID_FORMAT_STR and SD_ID128_MAKE_UUID_STR(), and we parse
UUIDs, hence add a high-level helper for formatting UUIDs too.

This doesn't add any new code, it just moves some helpers
id128-util.[ch] → sd-id128.[ch], to make them public.