feat: add sunshine setup

[maisatanel]

Sunshine is a host server for streaming applications, kinda like Steam Link but with less latency. It is used via the client Moonlight. Installation docs: https://docs.lizardbyte.dev/projects/sunshine/en/latest/about/installation.html#linux To make it work with Wayland, Sunshine has to be run as root. To make it work in Steam Game Mode, it has to be autostarted. What I've done here:

• Regrettably, pulseaudio-utils has to be layered, because Sunshine uses pactl.
• I adapted the postinstall script (upstream) to create a system service so that the app runs as root. Udev rules aren't installed since they're part of ublue anyway.

Issues:

• The systemd service won't autostart on boot. I don't know why, it starts just fine. Autostart is crucial for getting it to work with Steam Game Mode.