Support copyrightHolder

[joshmoore]

From today's challenge meeting, if the license is CC-BY, then having information on the copyright holder might be important. The proposal is not to make it a required field, but to add support for it.

An example from https://www.researchobject.org/ro-crate/specification/1.1/contextual-entities.html#licensing-access-control-and-copyright :