JWT token returned by the Identity Provider should include a custom claim for the policy, this is required to create a STS user in MinIO. The name of the custom claim could be either `policy` or `<NAMESPACE_PREFIX>policy`.  If there is no namespace then `claim_prefix` can be ignored. For example if the custom claim name is `https://min.io/policy` then, `claim_prefix` should be set as `https://min.io/`.