Support service definitions with descriptors

[evaline-ju]

Is your feature request related to a problem? Please describe.

As a follow-up to the JSON to service issue, we may want to support Descriptors directly in the JSON service, rather than just the input/output types for the RPC methods.

This would allow accepting direct references like in jtd to proto but does not currently work directly with jtd.validate

Describe the solution you'd like

Being able to define services with descriptors like the following:

{
        "service": {
            "rpcs": [
                {
                    "name": "FooPredict",
                    "input_type": InputDescriptor,
                    "output_type": OutputDescriptor,
                }
            ]
        }
    }

Describe alternatives you've considered

Current support, without descriptors, just types

jtd.validate validation errors could be caught and parsed e.g. ValidationError(instance_path=['service', 'rpcs', '0', 'output_type'], schema_path=['properties', 'service', 'properties', 'rpcs', 'elements', 'properties', 'output_type', 'type'])]