feat: Add data URI support for @image-url() function

[michneaionuc]

• Add base64 and urlencoding dependencies
• Extend resolving pass to detect and process data URIs
• Create temporary files for integration with existing embed_images pass
• Support PNG, JPEG, GIF, WebP, and SVG formats
• Closes Support for data uri (base64) in @image-url #4905

[CLAassistant]

All committers have signed the CLA.

[ogoffart]

Thanks a lot for looking into this and contributing.

You could also add tests.
There is one test there that can be re-used: https://github.com/slint-ui/slint/pull/7201/files#diff-534717e5e2d4787e61ae9e281e0ee44de584b9bc27caf6818d75ed317a1c4fd0
Also add a screenshot test: https://github.com/slint-ui/slint/blob/master/docs/testing.md#screenshot-tests

The idea to store in a temporary file is great. Although i think we could improve it so we don't need it. You could add another variant the enum ImageReference for this kind of data, and either process it in the passes/embed_images.rs in a new kind of resources, or just keep it raw so it can be handled in the generated code just like we load other ressources.