libtaskmanager: Add outputName to ScreencastingRequest
We need the ability to record a screen by name for the Plasma Mobile screen recording quicksetting: https://invent.kde.org/plasma/plasma-mobile/-/blob/master/quicksettings/record/contents/ui/main.qml#L50
This code was taken from the KPipewire tests.
Fixes plasma-mobile#207 (closed)
CC @apol