rgw/sts: fix read_obj_policy permission evaluation

rgw/sts: fix read_obj_policy permission evaluation
to pass in boost::none for the identity parameter
as identity IAM policies do not have a Principal
for evaluation.
The Principal is the role or the identity to which
the policy is attached.

Also removing boost::optional<const rgw::auth::Identity&> id
paremeter from eval_identity_or_session_policies in all
places, since an identity or a session policy doesnt have
a Principal element. The identity (user or role) or the
session is implicitly the 'Principal' to which the policy
is attached.

fixes: https://tracker.ceph.com/issues/52302

Signed-off-by: Pritha Srivastava <prsrivas@redhat.com>