Add files for KDE JSON schemas

This will allow consumers to reference the JSON schemas for the KPluginMetaData spec, and more specific usecases like Applets or KRunner plugins.

Apart from validation that correct keys are used, it provides better autocompletion and thus makes API features easier to discover.

This goes hand-in-hand with improving the JSON validation on the CI: sysadmin/ci-utilities!314

To fully implement like a project local jsonschema that extends the KPluginMetaData spec, this is a prerequisite.