RFC 0056: Service decorator needs a mandatory did field

Suppose I receive a request with a service decorator as shown at https://github.com/hyperledger/aries-rfcs/tree/527849ec3aa2a8fd47a7bb6c57f918ff8bcb5e8c/features/0056-service-decorator#tutorial and need to send a response message. What should be in the ~thread.received_orders field of the response as shown at https://github.com/hyperledger/aries-rfcs/tree/64e5e55c123b2efaf38f4b0911a71a1c40a7f29d/concepts/0008-message-id-and-threading#threaded-messages ? It is supposed to be of the form {"":1}, so where do we get the sender's DID in this case since the service decorator does not have a DID?

To solve this, I propose that we add a mandatory `did` field to the service decorator. Thoughts?