NUT-00: Invent way to request back change in a token

when we send someone a token from a foreign mint that supports bolt11, it could be that the receiver ends up with overpaid fees that they don't want to keep. they could send it back to the payer.

the payer could include a payment request in either the token or somehow pass a payment request with the payment (I think it would be cleanest to have it all in the same object, i.e. the token).