"There are three possible ways for providers to avoid application of the anti-assignment provisions in plan documents. One way is for providers to act as the patient's 'authorized representative.' ... A second way to avoid application of anti-assignment provisions in ERISA plans is to have the patient sign a limited power of attorney, allowing the hospital to pursue the patient's rights in the name of the patient.... A third way to avoid application of an anti-assignment clause in an ERISA plan is when the plan waives the anti-assignment provision by paying the provider directly. "