Rewrite the DSA-lobby backend in Rust

[Ma27]

To have a consistent code-base in the long run, it has been decided on our last sync a few weeks ago that the game-lobby API (currently written in C# and available in the old DiscoBot repo) should be ported to Rust as well.

With this change, we'll have even less components cluttered across several repos and can build (almost) the entire project in one run.