Refactor online_image to async_image and allow alternative data sources

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

The online_image component is a useful feature for loading images from a URL. I'm building a component for reading data from an SD card and I'd like to be able to reuse most of the logic in online_image but for reading from SD instead. I think it would be a good pattern to have a async_image platform component which could have various image sources plugged in - online_image, sd_image, etc.

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

This would allow alternative image data sources to be added to ESPHome with less overhead / copying of code.

**Additional context**