rights extensions are actually just registrations

[azaroth42]

In the Presentation API, it says:

The value must be drawn from the set of Creative Commons license URIs, the RightsStatements.org rights statement URIs, or those added via the extension mechanism.

However you don't add a new URI via a context like this. The context would be needed if you alias it to some string, in the way we do with motivations in WADM or behaviors in IIIF, but not when it's the full URI.

I think what we meant was "by registering the URI and the expected client behavior in the Rights registry" ... which doesn't exist, perhaps because no one has tried to do this ... until now.

Propose to make this change for 4.0

(Thanks to @alliomeria for pushing on this!)

[alliomeria]

Thanks for opening this related issue @azaroth42!