Introduce foreigner network (#25)

* support network identity for instance

* introduce foreign network

foreign network allow a node serving as one public node. other nodes can
connect to this node to discover peers and exchange route info.

easytier-core/src/peers/mod.rs

@@ -8,6 +8,9 @@ pub mod peer_rpc;
 pub mod route_trait;
 pub mod rpc_service;
 
+pub mod foreign_network_client;
+pub mod foreign_network_manager;
+
 #[cfg(test)]
 pub mod tests;