Add `payload` to validation output

[kMutagene]

We need a mechanism for validation package output containing arbitrary json payloads, so a downstream ARC app can be triggered and process the result accordingly.

Current proposal is just adding a payload field to validation_summary.json, while on the data model accepting a json object.

[omaus]

I'm interested in this. How will the payload field look like? If I got it correctly, it's a JSON object, how does this look like in the implementation? Serialized as string?

[kMutagene]

I have already discussed this with @j-bauer and am implementing a POC right now. It is just a Dictionary<string,obj> so any payload can be generated in a validation package.