Support for length param when sending to avoid allocating byte array.

[johnou]

We can exploit the fact the WebGL implementation takes a byte array and length ( instance.ws.send(HEAPU8.buffer.slice(bufferPtr, bufferPtr + length)); ) and allow the application to write to a buffer then pass that into the websocket send function instead of always allocating a new byte array eg.

int outputBytes;
while ((outputBytes = _tls.ReadOutput(_outgoingBuffer, 0, _outgoingBuffer.Length)) > 0)
{
  _socket.Send(_outgoingBuffer, outputBytes);
}

[johnou]

ptal @jirihybek cc @endel

[johnou]

@jirihybek poke, wdyt?