Calibration data storage (e.g. for power monitoring plugs)

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

I'm trying to create a firmware that could be pre-flashed on power monitoring smart plugs. They all require individual calibration, and that data needs to be able to survive a firmware update. At present, this has to be hard coded into the yaml (can be via substituions), but if a device is `dashboard_import`ed, the yaml it downloads from the repo will just have the default (uncalibrated) values. It would be very helpful for there to be support for a way in which calibration data could be stored in such a way that that data could survive future package/firmware updates. In my experience, flashing an updated esphome firmware seems to clear/reset global variables even when restore_from_flash is set. Does this need a new separate scheme for "calibration data"?

**Please describe your use case for this integration and alternatives you've tried:**
A way to reliably store calibration data, that won't be erased when an updated firmware is installed.

The issue names no source files, tests, or entry points; start by reviewing ESPHome's existing global-variable and flash-restore behavior described in the request. Done would mean calibration data survives firmware or package updates and is not replaced by default values after dashboard_import.

Written by the indexing model from the issue text.