Restore multi-version support in BeatUpClient

[rcelyte]

BeatUpClient 0.6.0 included incompatible changes to facilitate Beat Saber 1.31.0 support. The server will need to continue providing protocol support for legacy BeatUpClient versions until an updated mod is available for all game versions 1.20.0+

• A more modular architecture would enable BeatUpClient to apply sets of patches conditionally depending on game version, as well as providing a path forward for porting to Quest one component at a time
  • Modularity will also improve general maintainability and debuggability in the long run
• Moving away from C♯-exclusives (delegates, Harmony, etc.) may improve performance moderately and ease development of the Quest mod (BeatUpClient's Detour interface is an example of this, mimicing the beatsaber-hook core mod)
• The BeatUpClient protocol desperately needs a rework (especially player->player downloads)