Consider normatively/non-normatively adding site display information in interaction URL response

[dlongley]

In the vein of #633 and helping to support DC API use cases, it might be good to specify an optional JSON key that can be included in the response from an interaction URL -- whose value is display information. This would mirror what is going into X.509 certificates today for approaches that do have the benefit of interaction URLs.

So for example:

{
  "protocols": {...}
  "newJSONkey": { /* something comparable to what is going into X.509 certs */ }
}