Add optional `lastchange` for sensor values

[xopr]

Most sensors have different intervals of providing their value (like every 5 minutes or once a day). Sometimes a sensor is disconnected, say, shut down manually, wireless problems, crashed or for whatever reason inconclusive. It would be handy to allow a lastchange field next to its value similar to the spacestate open value.

A sensor example would look as followed:

        "power_consumption": [
            {
                "value": 0,
                "unit": "W",
                "location": "hackspace",
                "lastchange": 1645017421,
                "name": "hackcorner",
                "description": ""
            }
        ],