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/instance/dns_server/mod.rs

@@ -0,0 +1,15 @@
+// This module is copy and modified from https://github.com/fanyang89/libdns
+pub(crate) mod config;
+pub(crate) mod server;
+
+pub mod client_instance;
+pub mod runner;
+pub mod server_instance;
+pub mod system_config;
+
+#[cfg(test)]
+mod tests;
+
+pub static MAGIC_DNS_INSTANCE_ADDR: &str = "tcp://127.0.0.1:49813";
+pub static MAGIC_DNS_FAKE_IP: &str = "100.100.100.101";
+pub static DEFAULT_ET_DNS_ZONE: &str = "et.net.";