splithttp: Add support for H2C and http/1.1 ALPN on server (#3465)

* Add H2C support to server

* update comment

* Make http1.1 ALPN work on SplitHTTP client

Users that encounter protocol version issues will likely try to set the
ALPN explicitly. In that case we should simply grant their wish, because
the intent is obvious.

transport/internet/splithttp/hub.go

@@ -19,6 +19,8 @@ import (
 	"github.com/xtls/xray-core/transport/internet"
 	"github.com/xtls/xray-core/transport/internet/stat"
 	v2tls "github.com/xtls/xray-core/transport/internet/tls"
+	"golang.org/x/net/http2"
+	"golang.org/x/net/http2/h2c"
 )
 
 type requestHandler struct {
@@ -268,16 +270,21 @@ func ListenSH(ctx context.Context, address net.Address, port net.Port, streamSet
 		}
 	}
 
+	handler := &requestHandler{
+		host:      shSettings.Host,
+		path:      shSettings.GetNormalizedPath(),
+		ln:        l,
+		sessions:  sync.Map{},
+		localAddr: localAddr,
+	}
+
+	// h2cHandler can handle both plaintext HTTP/1.1 and h2c
+	h2cHandler := h2c.NewHandler(handler, &http2.Server{})
+
 	l.listener = listener
 
 	l.server = http.Server{
-		Handler: &requestHandler{
-			host:      shSettings.Host,
-			path:      shSettings.GetNormalizedPath(),
-			ln:        l,
-			sessions:  sync.Map{},
-			localAddr: localAddr,
-		},
+		Handler:           h2cHandler,
 		ReadHeaderTimeout: time.Second * 4,
 		MaxHeaderBytes:    8192,
 	}