Payment method to use for following payments during and post reactivation.
If unset, the subscription's existing default_payment_method is preserved.

Behaviour by collection_method:

• CHARGE_AUTOMATICALLY: a fresh invoice is finalized and immediately auto-charged using this PM
  (or the existing default if unset). Reactivation requires a non-empty PM.
• REQUEST_PAYMENT: a fresh invoice is finalized but NOT auto-charged; the customer pays it
  out-of-band. The PM is optional here and stored on the subscription for future flips/use.