Add SMTP mail server to send notifications

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

Email notification as backup for HA notifications when HA goes down.

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

I use home-assistant (supervised) for notifying me about issues on services that are running on the server or other network devices. But when The whole server freezes up or when HA suddenly is down I'm not notified about that.
Last weekend I noticed that the server that is also hosting Homeassistant freezed up and was not working anymore (for unclear reasons) untill i rebooted the system. If i could use a esphome node to check the status of the server I could send myself an email notification when the server is down.

**Additional context**

The issue names no files, tests, or entry points. Start by clarifying the SMTP configuration, triggering behavior, and how an external ESPHome node would detect that Home Assistant or its host is unavailable. Define acceptance criteria for sending a backup email notification before implementation begins.

Written by the indexing model from the issue text.