Sensor for modbus device state

**Describe the problem you have/What new integration you would like**

I would like to be able to check the state of a modbus device. Just seeing if the device is online or offline, like ping for webservices.

**Please describe your use case for this integration and alternatives you've tried:**

I am currently monitoring my solar inverter through modbus. All working well, until the sun goes down and the inverter shuts off. The state of my modbus sensors will keep showing the latest known value received from the inverter, so it looks like the inverter is still delivering very little power all night long. Would be nice if I could either have a sensor for the modbus state of the device which I can use to overwrite the sensor states to 0, or have something like the availability payload options for mqtt sensors in home assistant for each modbus sensor.

**Additional context**

The request names no files, tests, or entry points. First inspect the ESPHome Modbus integration and its handling of unavailable devices, then determine whether a device-state sensor or per-sensor availability is feasible; done means inverter-backed sensors no longer retain stale values when the device is offline.

Written by the indexing model from the issue text.