feat: json2 schema & tests

[terriko]

• related: feat: new json format for output #3980

In #3980, @mastersans has added a new json export format for cve-bin-tool (named json2 if you want to try it). I'd like to have a schema for it and a test that runs jsonschema.validate or equivalent on a basic output and on our test data so that once we settle into a schema we don't accidentally break things.

I'd consider this schema to be in flux until the 3.4 release, but we're hoping to have a pre-release for 3.4 later this month so we'll likely be settling on the final form soon.

[anthonyharrison]

see also #4333

[terriko]

Okay, we've got the description in the docs. I'd still like a schema and tests, but I think I'm going to accept that this isn't happening in time for 3.4 and move it out to the "future" milestone.