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/src/web_client/mod.rs

@@ -0,0 +1,48 @@
+use std::sync::Arc;
+
+use crate::{common::scoped_task::ScopedTask, tunnel::TunnelConnector};
+
+pub mod controller;
+pub mod session;
+
+pub struct WebClient {
+    controller: Arc<controller::Controller>,
+    tasks: ScopedTask<()>,
+}
+
+impl WebClient {
+    pub fn new<T: TunnelConnector + 'static, S: ToString>(connector: T, token: S) -> Self {
+        let controller = Arc::new(controller::Controller::new(token.to_string()));
+
+        let controller_clone = controller.clone();
+        let tasks = ScopedTask::from(tokio::spawn(async move {
+            Self::routine(controller_clone, Box::new(connector)).await;
+        }));
+
+        WebClient { controller, tasks }
+    }
+
+    async fn routine(
+        controller: Arc<controller::Controller>,
+        mut connector: Box<dyn TunnelConnector>,
+    ) {
+        loop {
+            let conn = match connector.connect().await {
+                Ok(conn) => conn,
+                Err(e) => {
+                    println!(
+                        "Failed to connect to the server ({}), retrying in 5 seconds...",
+                        e
+                    );
+                    tokio::time::sleep(std::time::Duration::from_secs(1)).await;
+                    continue;
+                }
+            };
+
+            println!("Successfully connected to {:?}", conn.info());
+
+            let mut session = session::Session::new(conn, controller.clone());
+            session.wait().await;
+        }
+    }
+}