Add flag to write output to JSON

[mattnappo]

This uses serde_json to write the output of a run as a json file.

[Hugoch]

Hey @mattnappo ! Thanks for your contribution, makes sense to have some JSON output for result parsing.
In your approach you are cloning the results and then building the summary, I think we can avoid cloning and rather build a global summary that can then be output via different writes (like ConsoleWrite, JsonWriter...).
It makes it more reusable and avoids cloning/computing twice the results.