Feature Request - Implement `Serializable` and `Deserializable` for `egui::Image`

**Is your feature request related to a problem? Please describe.**
For my use-case, I need to be able to (de)serialize images. I'm fine with serializing just the bytes, but there's no way (that I know of) to get the bytes of the image.

**Describe the solution you'd like**
Implement `Serializable` and `Deserializable` for `egui::Image`, or at least for `egui::ImageSource`.

**Describe alternatives you've considered**
Alternatively, just create a `.get_bytes()` function in `egui::ImageSource`. I can then (de)serialize that and construct a new Image. The other approach would be more convenient tho.

**Additional context**
This is due to the deprecation of RetainedImage. Prior to this, I created a RetainedImage from a ColorImage, which I created from a `&[u8]`. Now there's no clear way of replicating that with `egui::Image`s.

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Thank you for your amazing work!

Start by locating the definitions of egui::Image and egui::ImageSource, then review how RetainedImage and ColorImage handled image bytes before their deprecation. Determine a supported serialization boundary and verify that serialized image data can reconstruct an equivalent Image or ImageSource.

Written by the indexing model from the issue text.