NixOS vars

NixOS vars allow declaring secrets that live inside (or outside) the nix store in a declarative manner. Vars already have a WIP implementation over at https://github.com/NixOS/nixpkgs/pull/370444. The project would involve me working with Lassulus (the author of said PR) on finalizing the implementation. The long term end goal is having an upstream solution to the issues tools like sops-nix and agenix are currently solving (all while remaining agnostic to the underlying format). After speaking to Lassulus, the goals I'd be working on are: - writing a good example backend implementation - thinking about and solving some remaining architectural questions (when do the vars get generated, and how that affects the system itself) - defining a clean & clear interface for third party backend implementations Answering the above questions would require studying existing solutions (clan-core, nixos-artifacts, agenix-rekey) and picking out the best ideas from each.