DPROD — Synchronize with ideas from Dataspaces

See https://github.com/EKGF/dprod/issues/86
Dataspaces is a huge thing in Europe and is used in numerous industries for sharing of commercial/protected (not open) data.
Dataspaces use odrl:Offer and then a very simple "contract negotiation" protocol. Please add an example along these lines:
https://docs.internationaldataspaces.org/ids-knowledgebase/v/dataspace-protocol/contract-negotiation/contract.negotiation.protocol