Add support for unsubscribing

[aj-michael]

The backend has supported unsubscribing for a while now. We need to add it to this SDK.

[robinsdm]

I don't think @danhanson has documented how to unsubscribe.

[aj-michael]

It's right here: https://github.com/CSSE497/pathfinder-server/blob/dev/app/io/pathfinder/websockets/WebSocketMessage.scala#L133

iOS has had it for more than a month I think

[aj-michael]

@danhanson Your turn! 🚀

[danhanson]

Add this to sprint 13