Add support for the referencing suite

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.

Start by reading the referencing-suite repository and the current Bowtie protocol entry points. Determine how the suite should execute against implementations, including whether issue #3 enables support without protocol changes or a native mode is needed. Done means the suite runs successfully against the supported implementations with any required protocol behavior documented and tested.

Written by the indexing model from the issue text.