Improve nix setup
Improves the overall Nix experience by:
- Moving the package derivations to a separate file
- Moving the nix shell derivation to a separate file
- Enabling build for multiple architectures (
aarch-darwin
,aarch64-linux
,x86_64-darwin
,x86_64-linux
) - Adding a nixosModule to run μD3TN as a systemd service in NixOS environments
- Adding an integration test that checks whether the nixos module is able to run multiple μD3TN instances on a host without conflicts