support nix >2.19

[raboof]

Our rust utilities currently rely on a libnixstore from harmonia, which links against nix_2_19.

This means the lila utilities work on nixos 24.05 but no longer on nixos-unstable, which since dropped 2.19

Unfortunately, harmonia has since removed the libnixstore component, so upgrading harmonia is not a solution.

There's several ways to resolve this situation:

• extract the relevant bits of libnixstore (that targets nix libstore via C++ code) into our utilities
• extract libnixstore from before it was removed from harmonia and maintain it (and publish to crates.io) independently
• extend the nix C API to support the bits we need (similar to C API: nix_store_open doc, add storedir, real_path NixOS/nix#12044), and then use FFI with that C API from our utilities

The last approach seems most attractive, as the C API will longer-term also be the basis for other bindings. We may be able to use/extract Rust bindings to the C API from nixops4

The APIs relevant to us appear to be:

• hash_path
• query_path_info
• sign_string
• Radix::Base32
• query_references

[JulienMalka]

I am also more favorable on binding to the C API because it is supposed to be stable and will thus require less maintenance.