Add timestamp for latest event received

[fblackburn1]

From https://community.hubitat.com/t/node-red-nodes-for-hubitat/34386/5329

One of the things I really like in the Home Assistant node-red nodes is that events show a timestamp on the node (in addition to the current value). Is this something that you would consider adding (even if optional)? Example: It is very helpful for troubleshooting as it eliminates the need to add debug nodes in many cases to verify that the event "fired".

On initialization, I suggest to not adding any timestamp