dir auths should advertise their AuthDirMaxServersPerAddr value
As suggested by nusenu, and endorsed by Sebastian, in https://lists.torproject.org/pipermail/tor-relays/2023-February/021014.html
We should have dir auths be transparent about their AuthDirMaxServersPerAddr setting. Right now it can be implicitly guessed at, by looking at how many relays they vote for on a given IP address, but we can do better.