support quic proxy (#993)

QUIC proxy works like kcp proxy, it can proxy TCP streams and transfer data with QUIC.
QUIC has better congestion algorithm (BBR) for network with both high loss rate and high bandwidth. 
QUIC proxy can be enabled by passing `--enable-quic-proxy` to easytier in the client side. The proxy status can be viewed by `easytier-cli proxy`.

easytier/src/common/global_ctx.rs

@@ -75,6 +75,8 @@ pub struct GlobalCtx {
     no_tun: bool,
 
     feature_flags: AtomicCell<PeerFeatureFlag>,
+
+    quic_proxy_port: AtomicCell<Option<u16>>,
 }
 
 impl std::fmt::Debug for GlobalCtx {
@@ -137,6 +139,7 @@ impl GlobalCtx {
             no_tun,
 
             feature_flags: AtomicCell::new(feature_flags),
+            quic_proxy_port: AtomicCell::new(None),
         }
     }
 
@@ -281,6 +284,14 @@ impl GlobalCtx {
     pub fn set_feature_flags(&self, flags: PeerFeatureFlag) {
         self.feature_flags.store(flags);
     }
+
+    pub fn get_quic_proxy_port(&self) -> Option<u16> {
+        self.quic_proxy_port.load()
+    }
+
+    pub fn set_quic_proxy_port(&self, port: Option<u16>) {
+        self.quic_proxy_port.store(port);
+    }
 }
 
 #[cfg(test)]