Adding a JSON schema

[asmeurer]

It would be useful to have a JSON schema for the JSON produced by this plugin. https://json-schema.org/

This is something that I plan on implementing because I need it for a project that is using this plugin. Is it something that you'd be interested in having upstreamed into this plugin?

I noticed that the report here is for the most part just JSON serializing the objects that come from pytest, so this would mean that it would have to be kept in sync with any upstream changes that are made to pytest. But even so, this is really something that is needed to properly validate the JSON reports produced by this plugin.