Make it possible to request a plugin configuration module programatically

Summary:
In KDE Connect we want to show a specific configuration module in some
occasions, having this new API would be handy in this regard.

Test Plan: Implemented KDE Connect side which works

Reviewers: Frameworks, KDE Connect, albertvaka

Reviewed By: KDE Connect, albertvaka

Subscribers: albertvaka

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D11683