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

rgw/sts: changing identity to boost::none, when role policy
is verified for putobj permissions, in case of renaming a
large file.

While renaming a large file, putobj is invoked as an intermediate
step, and role policy is verified for the source object if temp creds
are used. Since the role policy is attached to the identity (role)
itself and the role policy does not contain a Principal, there is no
need to verify the identity and hence boost::none is passed in place
of the identity.

fixes: https://tracker.ceph.com/issues/58628

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