[design tokens]: Adding images through brand packages

Description

Currently, when there is a need to add images via CSS (background-image) and the corresponding design token that generates the corresponding CSS change, there is a problem with relative links. Consumers can generate CSS variables that will reference an image that is in the dist theme/brand package (relative link). But this link will change when the CSS variable is used in the MFE. It will be relative to the MFE (React app).

For example:

Design token in elm-theme:

  "catalog-home-page-banner-background-image": {
    "$value": "home-bg.jpg",
    "$type": "asset",
    "$description": "Catalog Home page background image."
  }

The path to home-bg.jpg relative to dist:

https://cdn.jsdelivr.net/gh/raccoongang/elm-theme@474e869cb03bb000c2d44f87bae2000c35826e6a/dist/home-bg.jpg

The path that gets var(--catalog-home-page-banner-bg) variable:

http://apps.local.openedx.io:1998/catalog/home-bg.jpg

More context: https://github.com/openedx/frontend-app-catalog/pull/7#discussion_r2228141749

Start with the design-token asset example and the linked frontend-app-catalog discussion. The issue names no files, tests, or entry points, and it does not define the expected implementation or acceptance criteria. Clarify how image URLs should resolve for consumers before choosing a code path.