MQTT Retain Flag

[lloydbayley]

💡 The Idea
Would it be possible to add a retain flag to the MQTT notification?

It would be handy to see what the last notification was before a reboot or close of subscription for any reason.
I suppose QOS could be done at the same time if you're in the code, but the retain flag would be a nice addition.

[skellycode]

I'd also love to see the retain flag added. It's a critical feature for many use-cases!