sdap: properly handle binary objectGuid attribute
Although in the initial processing SSSD treats the binary value right at
some point it mainly assumes that it is a string. Depending on the value
this might end up with the correct binary value stored in the cache but
in most cases there will be only a broken entry in the cache.
This patch converts the binary value into a string representation which
is described in [MS-DTYP] and stores the result in the cache.
Reviewed-by: Jakub Hrozek <email@example.com>