Introduce PluginManager

This adds a dedicated class for loading/unloading and sending data to a plugin rather than doing it all in main().
It also keeps track of the loaded plugins and their supported version which the extension will query again in the future.
It could also potentially handle things like sending a reply to an incoming dataset by storing a reference/identifier to the request etc.

Differential Revision: https://phabricator.kde.org/D22065