feat: add nix packages and dev shell

[elsirion]

Description

This PR adds a nix flake with packages for the iroh-dns-server and iroh-relay binaries as well as a rust dev shell for development. To make the packaging work better (less redundant versions in nix flake) I also made the crate versions a workspace property that is inherited.

Breaking Changes

None to my knowledge.

Notes & open questions

• Should the .envrc file be part of this PR? It automatically activates the dev shell via direnv if set up and allowed.
• Should the cargo changes be split into a separate commit?
• Does the nix dev shell need docs? I'd just assume fellow nixers will recognize it being there and be happyy while others will keep doing whatever they were doing before.
• Longer term, out of scope here imo: should more things like dependencies be moved to the workspace (can recommend, we did that with Fedimint
  and it made changes much more manageable)

Change checklist

• Self-review.
• Documentation updates following the style guide, if relevant.
• Tests if relevant.