feat: add watch-exec command

[TECHNOFAB11]

This builds on top of the changes in #21 and adds the watch-exec command which works the same like the cachix CLI one. I added counters to track the pending and currently running upload jobs, the watch-exec command basically just starts the daemon, runs the passed command with a custom nix config which adds attic queue relay as a post-build-hook and then waits until the mentioned counters go to 0. Haven't used Rust in a while so suggestions and improvements welcome :)