Add support for streaming transfers

This change adds onChunkCallback to the request. If defined it will be
called repeatedly with the incoming data. This allows to process data on
the go or write it to disk instead of accumulating the data in memory.

Submitted upstream:
https://github.com/machinezone/IXWebSocket/pull/353

extern/IXWebSocket-11.3.2/ixwebsocket/IXHttp.cpp

@@ -149,7 +149,7 @@ namespace ix
                     false, "Error: 'Content-Length' should be a positive integer", httpRequest);
             }
 
-            auto res = socket->readBytes(contentLength, nullptr, isCancellationRequested);
+            auto res = socket->readBytes(contentLength, nullptr, nullptr, isCancellationRequested);
             if (!res.first)
             {
                 return std::make_tuple(