TUN-4596: Add QUIC application protocol for QUIC stream handshake

- Vendored the capnproto library to cloudflared.
- Added capnproto schema defining application protocol.
- Added Pogs and application level read write of the protocol.

quic/schema/quic_metadata_protocol.capnp

@@ -0,0 +1,28 @@
+using Go = import "/go.capnp";
+@0xb29021ef7421cc32;
+
+$Go.package("schema");
+$Go.import("schema");
+
+
+struct ConnectRequest{
+	dest @0 :Text;
+	type @1 :ConnectionType;
+	metadata @2 :List(Metadata);
+}
+
+enum ConnectionType{
+	http @0;
+	websocket @1;
+	tcp @2;
+}
+
+struct Metadata {
+    key @0 :Text;
+	val @1 :Text;
+}
+
+struct ConnectResponse{
+	error @0 :Text;
+	metadata @1 :List(Metadata);
+}