[nix] add tool to identify unused dependencies in Cargo projects

Summary:
we can run cargo-udeps in cargo projects to identify and prune unused dependencies.

i'm not sure why we have to add the program in nativeBuildInputs, because it seems to work fine in my nix shell even without it there.

Test Plan: ran cargo-udeps in my nix shell successfully

Reviewers: jon, O1 Nix

Reviewed By: jon, O1 Nix

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7247