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/proto/common.proto

@@ -35,6 +35,11 @@ message FlagsInConfig {
   bool accept_dns = 22;
   // enable private mode
   bool private_mode = 23;
+
+  // should we convert all tcp streams into quic streams
+  bool enable_quic_proxy = 24;
+  // does this peer allow quic input
+  bool disable_quic_input = 25;
 }
 
 message RpcDescriptor {
@@ -171,3 +176,7 @@ message PortForwardConfigPb {
     SocketAddr dst_addr = 2;
     SocketType socket_type = 3;
 }
+
+message ProxyDstInfo {
+  SocketAddr dst_addr = 1;
+}