The imeptus for this is to allow embedding an event timestamp with the
data, since MQTT doesn't tag messages with the time they were sent. So
Instead of having a payload value like "some value", you could have:
{
"timestamp": 1630360029124,
"value": "some value"
}
... and then configure the mapping in mqtt2db to be a JSON mapping, with
timestamp field name "timestamp" and value field name "value", and we
just figure things out.
If no mapping payload type is specified, we just assume it's a raw value
as before.
