allow adding PIDs for workflows content types

[laureD19]

registered prefix for Marketplace

backend to request a PID for a resource to the handle server, at the stage of the approved status of workflows.
Stored in the metadata of a workflow item.

@vronk - can you add details here about the handle server, please?

[laureD19]

during the creation/publication of workflow, make a call to the Handle server and ask for a PID
Add the PID from the Handle to externalID attribute, with the "Handle" type.

Contact point to get the endpoint and the credentials: @vronk or @dpancic