Paho client doesn't support custom headers for MQTT connection over WebSockets

When you need connection to MQTT broker using WebSockets, paho client sets default headers:
```
pw.print("Upgrade: websocket" + LINE_SEPARATOR);
pw.print("Connection: Upgrade" + LINE_SEPARATOR);
pw.print("Sec-WebSocket-Key: " + key + LINE_SEPARATOR);
pw.print("Sec-WebSocket-Protocol: mqtt" + LINE_SEPARATOR);
pw.print("Sec-WebSocket-Version: 13" + LINE_SEPARATOR);
```

In some cases required send custom headers, for example if you wanted to connect to AWS Iot broker.
https://docs.aws.amazon.com/iot/latest/developerguide/custom-auth.html

It would be great add custom headers support to WebSocket handshake request.

- [ ] Bug exists Release Version 1.2.0 ( Master Branch)
- [ ] Bug exists in Snapshot Version 1.2.1-SNAPSHOT (Develop Branch)
- [ ] Bug exists in MQTTv5 Version (mqttv5-new Branch)

If this is a bug regarding the Android Service, please raise the bug here instead: https://github.com/eclipse/paho.mqtt.android/issues/new