Add custom system-manager module

[sjmonson]

system-manager is a framework for getting a NixOS like experience on non-NixOS Linux systems. System-manager enables a subset of NixOS modules which currently does not include nix.registry or any management of the system-wide Nix version. This PR adds a module that re-implements some of the missing functionality with some caveats.

Bootstrapping Nix

There is a chicken vs egg problem with respect to bootstrapping the nix-daemon. The best way to solve this is to:

1. Install nix with the determinate nix-installer
2. Run system-manager switch with this module enabled and confirm it attempts to replace service/sockets
3. Delete the Nix service/sockets but do not reload systemd
4. Rerun system-manager switch

SELinux

SELinux is currently not supported by system-manager, though there is a hacky workaround.

nixbld User Management

The nixbld users are currently not implemented in this module. system-manager does offer user management; however due to the bootstrapping issue the users will already be available and managing them seems like an unnecessary complication.

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