rgw/kms/vault - share get/set attr between rgw_crypt.cc and rgw_kms.cc
In order to pass down and manage "attrs" from crypt logic to kms
logic, it's necessary to share the functions that can get and
set strings in that structure. Eventually, I plan to have
the various engines store and retrieve a per-object "datakey" that
is encrypted (wrapped) by the named kms key.
Fixes: http://tracker.ceph.com/issues/48746 Signed-off-by: Marcus Watts <mwatts@redhat.com>