// parent is derived as a concatenation of the `sub` and `iss` fields. The
// policies applicable to the STS credential are associated with this "virtual"
// parent.
//
// When a policyName is given to this function, the policy association is
// created and stored in the IAM store. Thus, it should NOT be given for the
// role-arn case (because the role-to-policy mapping is separately stored