“Specifically, we are concerned about proposals for open-ended arbitration, which have been floated as a solution to the problem. If arbitration appears innocuous, it is to a large extent because it is not transparent. Experience suggests that arbitration would be cumbersome to deploy, and highly favorable to those health care providers who charge high prices today. If Congress were to endorse arbitration, it could potentially open the door to a system quite unintended – establishing an inflationary dynamic that accommodates and encourages the rapid growth of costs.”