Sensor expiry and timestamps

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

Can't access sensor availability state or time of last update (i.e. when a measurement was taken). I could capture timestamp in `add_on_state_callback`, but this bulky and still doesn't provide information about expiry.

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

I'd like to display sensor readings (battery soc, temperature,...) and indicate if these readings are still valid.

**Additional context**

A timestamp field in the Sensor class would be awesome.

The request names the Sensor class and add_on_state_callback, but no repository file or test. Start by locating those entry points and clarify how measurement timestamps and expiry should be represented. Done means sensor readings expose when measurements were taken and whether they remain valid.

Written by the indexing model from the issue text.