Add ADC Vref as sensor 4 / Add timestamp to mqtt

[Zitt]

Add ADC Vref as sensor 4.
Allows the Vref to be measured and recorded as a constant for more accurate temperature sensor measurements.

Add Timestamp to mqtt messages
for broker and consumers of the data stream to "know" how old the data is on the broker.

[tjko]

@Zitt, thanks! I like the idea of showing/logging the Vref for ADC.

One minor thing is that the new command seems bit confusing, when its under "sensors". Maybe it should be under "SYS" as it is systemwide setting (and not per sensor setting). Perhaps, make it something like SYS:VREFadc.

For the MQTT change, I'd prefer not have spaces in JSON keys to keep things consistent, sso maybe instead of "date time", use "datetime" or "date_time"?

I can take your changes and make some tweaks, or do you want to update the pull request?

[Zitt]

@tjko I'm far from an expert; honestly, this was more of a hack but figured I'd share if it helps the project long term.

The Vref issue came about from the fact that my hardware "change" didn't fully understand how/why you used a 3.0 shunt regulator. I ended up specing a 3.3vref. In hindsight; I'd just change the hardware but given that's the only problem I've found there isn't a large ROI.

So; I'd prefer you make the changes you like in the spirit of your project.

Yes a space in the string is kinda silly I'd vote "datetime" but whatever you think is best.