Add Nix flake for mcproxyctl

Vendor dependencies and expose mcproxyctl binary via nix build.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

vendor/github.com/google/uuid/CONTRIBUTORS

@@ -0,0 +1,9 @@
+Paul Borman <borman@google.com>
+bmatsuo
+shawnps
+theory
+jboverfelt
+dsymonds
+cd1
+wallclockbuilder
+dansouza