mc_rtc::Schema should support generating JSON Schema files

[arntanguy]

To make mc_rtc::Schema truly useful, we should make it generate JSON Schemas automatically.
This would enable the framework code itself to export its expected properties, and make it easy to generate documentation for downstream projects as well (plugins, custom controllers, observers, etc). Once done we would need to spend the effort to convert all uses of mc_rtc::Configuration as mc_rtc::Schema. Most of the machinery for this is already in place, but it'll be quite the grind to get there.

This would play nicely with #483