Make it possible to retain published MQTT messages

[Scrin]

This is useful for Home Assistant users who use a long update interval (ie. a couple of minutes), allowing the tags to appear ~immediately after restarting Home Assistant. However this will also prevent automatically un-discovering tags if they are ever removed, requiring manually deleting the retained messages in such case. This should be configurable, and default to the current behavior of not using retained messages.

[jkaart]

I just hard coded this feature in my fork test branch.