Replacing jsonschema.RefResolver with referencing.Registry

[SarahJohnsonONS]

Results of the investigation into #854 as follows:

• jsonschema.RefResolver is currently being used in two places - models/jsonvalidationerrors.py and utils/json.py.
• In utils/json.py, RefResolver is used in the function resolve_path - however, this does not appear to be used anywhere in the codebase - when commented out, all unit and behaviour tests still pass.
• In models/jsonvalidationerrors.py, RefResolver is used in the functions _child_error_messages_display_string and _resolve_reference_in_schema of the AnyOneOfJsonSchemaValidationError class.
• Having installed the referencing package, some changes were made to models/jsonvalidationerrors.py (see branch).
• The referencing library documentation on migrating from RefResolver was followed, and some use cases have been implemented.
• The main issue that needs to be resolved is accommodating $ref values that are URIs - see the comments in models/jsonvalidationerrors.py L191-198 in the branch linked above. As noted, this is an issue for inline codelists, but there may be other examples that have not yet been identified.

Further spike/investigation on this issue:

• What would happen if we removed this functionality? What impact does it have and is there another way that can avoid using this method?
• Try finding other open source projects that have dealt with this deprecation issue, closed issues on GitHub.