Implement websocket support for mqtt

Implement websocket support for mqtt. Currently mqtt does not work for webassembly since neither QTcpSocket, nor QSslSocket works for webassembly.

There is a workaround spreading around that can be used, but it is better to actually have the workaround together with the mqtt code.

The fix is to take the existing workaround and package it with mqtt, in a binary compatible way.

While earlier it was necessary to create a QIODevice together with the required boilerplate code, it is now sufficient to do:

QMqttClient::connectToHostWebSocket[Encrypted]

mqtt itself is a publish/subscribe message protocol used by distributed systems. Some of our distributed examples for instance, like the outrun demo, uses this.

The implementation is done by creating two QIODevices that can be used for websockets and secure websockets.