Add support for the referencing suite

[Julian]

Add support for executing https://github.com/python-jsonschema/referencing-suite against implementations.

If python-jsonschema/referencing-suite#3 can be done this can be supported without protocol changes.

But given there's some trickiness there, and that even if it can be done we still may want a "native" mode, I think we should assume something may be needed in the protocol, even if not at first.