Binary sensor: Motion detected by camera

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

I would like to be able to act (in HA) upon anything moving in front of the camera.

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

I have checked the ESPHome and HA documentation and did not find anything that helps: the camera implementation in HA is really limited, and besides I don't think this belongs outside the ESP32.
I have seen some people exploring the concept: https://eloquentarduino.github.io/2020/01/motion-detection-with-esp32-cam-only-arduino-version/
I have seen someone comment on the binary sensor being there: https://www.reddit.com/r/homeassistant/comments/bf1zrw/my_experience_with_esphome_on_home_assistant/elfrfrk/
...but I can't find any way to enable such a binary sensor neither in ESPHome or HA.

**Additional context**

I am aware that other (advanced) processing features such as face detection or QR reading have been discarded in #341 , but firstly motion detection was not covered in that request, and secondly motion detection seems (naively) a less complex problem to deal with, and one that would justify (to avoid using bandwidth when not needed) the local processing.

Technical challenge: Would it be possible to do motion detection on the encoded MJPEG stream without fully decoding it?

No source files or tests are named. Start by reviewing the ESPHome camera implementation and the discussion in issue #341, then determine whether motion detection belongs in the ESP32 camera component; done would mean a documented binary sensor that reliably reports motion and can be used in Home Assistant.

Written by the indexing model from the issue text.