rgw/kms/vault - share get/set attr between rgw_crypt.cc and rgw_kms.cc

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>
(cherry picked from commit 89959fb1946c82e48144ba29f9587932730c396b)