Direct Methods only support map[string]interface{}

[z-br]

Azure, and specifically IOT Central, supports numbers/strings/bools as direct method payloads, which don't work in the marshaling layer here. One option would be to defer marshal/unmarshal to the implementer of the DirectMethodFunc. Maybe this could be handled in conjunction with #23 - there could be something like RegisterDirectMethodAdvanced that support asynchrony and did bytes in, bytes out.