SplitHTTP client: Add xmux (multiplex controller) for H3 & H2 (#3613)

https://github.com/XTLS/Xray-core/pull/3613#issuecomment-2351954957

Closes https://github.com/XTLS/Xray-core/issues/3560#issuecomment-2247495778

---------

Co-authored-by: mmmray <142015632+mmmray@users.noreply.github.com>

transport/internet/splithttp/connection.go

@@ -11,6 +11,7 @@ type splitConn struct {
 	reader     io.ReadCloser
 	remoteAddr net.Addr
 	localAddr  net.Addr
+	onClose    func()
 }
 
 func (c *splitConn) Write(b []byte) (int, error) {
@@ -22,6 +23,10 @@ func (c *splitConn) Read(b []byte) (int, error) {
 }
 
 func (c *splitConn) Close() error {
+	if c.onClose != nil {
+		c.onClose()
+	}
+
 	err := c.writer.Close()
 	err2 := c.reader.Close()
 	if err != nil {