Add a class registry

[Panaetius]

Right now you need to specify a schema to deserialize jsonld. Ideally, we could just throw jsonld at the library and it'd figure out which schema applies by itself by looking up the type in a registry.

I wouldn't do automated adding to the registry on import like current renku-python does it, it should be done manually in code in a single place to not have to deal with things not being loaded.