Ability to add historical data to Home Assistant using `async_add_external_statistics`

**Describe the problem you have/What new integration you would like**
Some ESPhome device do not have permanent connection to Home Assistant. Currently, when the connection to HA is lost, data from sensors is lost and will never be recorded in Home Assistant.

**Please describe your use case for this integration and alternatives you've tried:**
* Home Assistant upgrades
* Upgrade of network components (central router, Powerline Wi-Fi access point, etc.)
* Mobile devices (e.g. grass mower) temporarily moving out of network range

**Additional context**
An integration can add historical data into Home Assistant using `async_add_external_statistics`, as described [here](https://community.home-assistant.io/t/import-old-energy-readings-for-use-in-energy-dashboard/341406/9) for example.

Start with the async_add_external_statistics API and the linked Home Assistant community discussion to understand how integrations currently import historical statistics. There are no files or tests named in the issue; done would mean defining and implementing a supported way for disconnected ESPHome sensors to add missed historical data to Home Assistant.

Written by the indexing model from the issue text.