Have an application that can cross-check if the promised platforms are correct

Summary:
In KDE Frameworks projects (and some others) we have a metainfo.yaml file
that tells us which platforms it's supposed to work on. This is never
checked other than by maintainers.
This tool notifies us about a framework saying that a platform is available
when it's not even part of our CI.

Test Plan:

kcompletion: Android is a disabled platform
kcontacts: FreeBSD is a disabled platform
kdewebkit: Linux is a disabled platform
kidentitymanagement: FreeBSD is a disabled platform
syntax-highlighting: Android is a disabled platform

Reviewers: Frameworks, bcooksley

Reviewed By: bcooksley

Subscribers: adridg, bcooksley, dfaure

Differential Revision: https://phabricator.kde.org/D9076