Records that a purchase was cancelled in the external commerce system and queues
revocation of the user's access to the product.

The operation is idempotent. Cancelling a purchase that is already cancelled, or one
in any other state, is accepted and has no further effect.

Cancellation is asynchronous, so fulfillment_status in the response will usually not
yet read cancelled. Poll GET /commerce/purchases/{purchase_id} to confirm that it
completed.