XHTTP: Add "stream-one" mode for client & server (#4071)

""Breaking"": Client uses "stream-one" mode by default when using **REALITY** ("stream-up" if "downloadSettings" exists)

transport/internet/splithttp/browser_client.go

@@ -14,6 +14,10 @@ import (
 // has no fields because everything is global state :O)
 type BrowserDialerClient struct{}
 
+func (c *BrowserDialerClient) Open(ctx context.Context, pureURL string) (io.WriteCloser, io.ReadCloser) {
+	panic("not implemented yet")
+}
+
 func (c *BrowserDialerClient) OpenUpload(ctx context.Context, baseURL string) io.WriteCloser {
 	panic("not implemented yet")
 }