KIO: port the URI filter plugins from KServiceTypeTrader to json+KPluginMetaData

Summary:
This makes it possible to run the unittest without installing the
plugins first.

One thing I don't understand is the need for toString().toInt(),
or in other words, why my json file with a numeric value gets turned
into a string value when using qtplugininfo to inspect the plugin:
User Data: {

"X-KDE-InitialPreference": "5"

}

Test Plan: make uninstall; bin/jobtest

Reviewers: apol, davidedmundson, arichardson, bshah

Reviewed By: bshah

Subscribers: vandenoever, elvisangelaccio, bshah, Frameworks

Differential Revision: https://phabricator.kde.org/D7533