Support specifying extensible property requirements and capabilities for transfer requests.

[jimmarino]

Code-to-data and other scenarios may require the consumer to send information to the provider, for example, a mechanism for processing data before it is transferred.

This can be done using the extensible properties of TransferRequestDto and its analog inDataRequest. The extensible data will then be accessible from a resource generator and provisioner.

Currently, however, there is no recommended way for providers to advertise the requirements for this extensible data. One way to do this would be to use the extensible properties of Artifact where a schema representation could be embedded.

This issue serves as a marker to explore how this should be done.