Add JSON as Output format for validation

[JakobDev]

appstreamcli validate --format lets you choose between 2 different formats: text and yaml. The YAML format is there to be parsed by other programs. I think it would make sense to add a JSON format as well. It should have exactly the same as the YAML, but as JSON. Many programming languages (e.g. Python which is often used for scripts) today already ship with a JSON parser. If you want to parse YAML with these languages, you need a use a extra library. Even if a language ships without a JSON parser, the tooling for JSON is far better than for YAML, because it is more common. So it would be a good idea to add a JSON format.

[ximion]

Would be nice, but I don't intend to add a JSON parser or JSON writer dependency to AppStream, and switching libyaml to JSON mode would be quite a pain to implement reliably.
So this feature isn't super likely to be added any time soon...