magic dns (#813)

This patch implements:

1. A dns server that handles .et.net. zone in local and forward all other queries to system dns server.

2. A dns server instance which is a singleton in one machine, using one specific tcp port to be exclusive with each other. this instance is responsible for config system dns and run the dns server to handle dns queries.

3. A dns client instance that all easytier instance will run one, this instance will try to connect to dns server instance, and update the dns record in the dns server instance.

this pr only implements the system config for windows. linux & mac will do later.

easytier/src/proto/mod.rs

@@ -4,6 +4,7 @@ pub mod rpc_types;
 pub mod cli;
 pub mod common;
 pub mod error;
+pub mod magic_dns;
 pub mod peer_rpc;
 pub mod web;