Add freedesktop-sdk project

freedesktop-sdk is the base for (all?) flatpak runtimes. Snap runtime is in progress too. See https://freedesktop-sdk.io/

The project is working towards the goal of having 100% reproducible builds. Currently around 11 elements left

You can check progress and related MR's at https://gitlab.com/freedesktop-sdk/freedesktop-sdk/-/issues/107