Adding Plasma Discover to Bazzite via Systemd Sysext

Instructions to add Plasma Discover package manager back into Bazzite using a Systemd Sys-Ext. Based on Travier's Fedora Sys-Ext work at https://travier.github.io/fedora-sysexts/ and relies on his base images on quay.

I'm really excited about the application of SysExts to bridge the gap many perceive in adopting atomic distros! This seemed like a fantastic solution to adding this tool back for those who want it, without the overhead of package layering