reef: rgw/sts: changing identity to boost::none, when role policy

[k0ste]

backport tracker: https://tracker.ceph.com/issues/59607

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backport of https://github.com/ceph/ceph/pull/49965 parent tracker: https://tracker.ceph.com/issues/58628

this backport was staged using ceph-backport.sh version 16.0.0.6848 find the latest version at https://github.com/ceph/ceph/blob/main/src/script/ceph-backport.sh