Add support for reading from PNG Alpha (Stealth PNG)

[EgorkaSanyok]

Is your feature request related to a problem? Please describe.
Some websites strip EXIF metadata from images when uploaded, thus losing the prompt. However, a solution to circumvent that was implemented, namely, encoding the prompt info into the PNG alpha channel of a generated photo. This is dubbed "stealth PNG". NovelAI employs this by default, in addition to regular EXIF metadata.

Describe the solution you'd like
Plugins for sd-webui and ComfyUI that encode prompt generation into the PNG alpha channel have been developed, and can be found here:
https://github.com/neggles/sd-webui-stealth-pnginfo
https://github.com/catboxanon/comfyui_stealth_pnginfo
Adding support for this feature would make the Diffusion Toolkit more complete, and wouldn't have to use those web tools in order to see the metadata of such images. It would also be useful to have a way to copy this information back and forth between it and the EXIF metadata tags of the file itself, for better preservation.

Describe alternatives you've considered
Currently, this data can only be read by the plugins mentioned above, the official NovelAI inspect tool (https://novelai.net/inspect) as well as this plugin that only works on 4chan (https://gist.github.com/catboxanon/ca46eb79ce55e3216aecab49d5c7a3fb)

Additional context
Here is an example image, generated with sd-webui and the stealth-pnginfo plugin, having its EXIF data removed.

Without the stealth-pnginfo plugin, the PNG info tab in the web-ui will not display anything. Installing it will then show the proper metadata