[kded] Port plugin metadata to JSON

Summary: Use the modern way of doing things

Test Plan: Check for Print Manager in kded KCM

Reviewers: #frameworks, dantti, #kf6, dfaure

Reviewed By: dfaure

Subscribers: dfaure, apol, kde-utils-devel

Maniphest Tasks: T11919

Differential Revision: https://phabricator.kde.org/D25233