transport: change *http2Client to interface ClientTransport (#7512)

Author: printchard

internal/transport/transport.go

@@ -241,7 +241,7 @@ const (
 type Stream struct {
 	id           uint32
 	st           ServerTransport    // nil for client side Stream
-	ct           *http2Client       // nil for server side Stream
+	ct           ClientTransport    // nil for server side Stream
 	ctx          context.Context    // the associated context of the stream
 	cancel       context.CancelFunc // always nil for client side Stream
 	done         chan struct{}      // closed at the end of stream to unblock writers. On the client side.