Allow register itself & items within to be cited by exposing them as Relaton (ISO 690) bibliographic items

[strogonoff]

Concerning register as a whole, in addition to making it easier to cite, this could allow us to use Relaton model in register metadata and add some structure to the currently messy approach of shoving everything in contentSummary. As an example, contributors with enabler or distributor role could be used to render the list of sponsoring organizations in theory.

It’s unclear yet how do deal with the overlap between 19135-1 register metadata spec (which declares things like “content summary” and register stakeholders) and Relaton (which declares bibliographic item with abstract/keywords/notes/… and contributors with their roles).