feat/web (Patchset 2) (#444)

This patch implement a restful server without any auth.

usage:

```bash
# run easytier-web, which acts as an gateway and registry for all easytier-core
$> easytier-web

# run easytier-core and connect to easytier-web with a token
$> easytier-core --config-server udp://127.0.0.1:22020/fdsafdsa

# use restful api to list session
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/sessions
[{"token":"fdsafdsa","client_url":"udp://127.0.0.1:48915","machine_id":"de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f"}]%

# use restful api to run a network instance
$> curl -H "Content-Type: application/json" -X POST 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f -d '{"config": "listeners = [\"udp://0.0.0.0:12344\"]"}'

# use restful api to get network instance info
$> curl -H "Content-Type: application/json" -X GET 127.0.0.1:11211/api/v1/network/de3f5b8f-0f2f-d9d0-fb30-a2ac8951d92f/65437e50-b286-4098-a624-74429f2cb839 
```

easytier-web/src/main.rs

@@ -1,3 +1,22 @@
-fn main() {
-    println!("Hello, world!");
+#![allow(dead_code)]
+
+use std::sync::Arc;
+
+use easytier::tunnel::udp::UdpTunnelListener;
+
+mod client_manager;
+mod restful;
+
+#[tokio::main]
+async fn main() {
+    let listener = UdpTunnelListener::new("udp://0.0.0.0:22020".parse().unwrap());
+    let mut mgr = client_manager::ClientManager::new();
+    mgr.serve(listener).await.unwrap();
+    let mgr = Arc::new(mgr);
+
+    let mut restful_server =
+        restful::RestfulServer::new("0.0.0.0:11211".parse().unwrap(), mgr.clone());
+    restful_server.start().await.unwrap();
+
+    tokio::signal::ctrl_c().await.unwrap();
 }