Can't add further contexts for object in Change Discovery API

[azaroth42]

The rules for extensions say that they have to come first before the main context (for good reason, we don't want to extensions to change basic functionality and mappings).

However the change discovery context has an explicit definition for object and target, which will then overwrite any extension that tries to change them. Further, it has definitions (for scoped contexts) for all of the activity types, so they can't be where the extension takes place either.

Thus the extension would need to make the assertion as a top level context that would be valid anywhere, even if the only thing that we want to add is a new set of classes for the type of the object resource.

I believe this is in scope of the original issue (https://github.com/IIIF/discovery/issues/82) for non-IIIF uses of the API