[ApplicationLauncherJob] Add way to launch KServiceAction

This offers convenience API for launching a specific KServiceAction on a KService.

Plasma has a few places (Task Manager, Kickoff, KRunner) where we deal with service actions. Right now they all pretty much call KRun::runCommand(action.exec()).
This new API ensures it gets the same capabilities (including URL expansion) and proper startup info (app icon bouncy cursor) as regular service launches.

Differential Revision: https://phabricator.kde.org/D28285