Add support for MQTT binary mode

[jskeet]

We've updated the dependency, so we should be able to do that now.

[jskeet]

This will be post-2.0.

[jskeet]

This requires us to know whether the client supports MQTT v5 or not, so there's a bit more API design work to be done. Given that no-one has requested it in 3 years, it may not be terribly important. I've got a part-implemented PR which I'll attach here though.