Add support for Asset tokens

[rbosker]

Description

Sometimes users want to reference an image to be replaced when switching between brands. As a first start, we should allow them to just point to a URL that we can then fetch.

Background

type: 'asset',
label: 'Assets',
appliedToken: 'asset',
value: The value will be a string pointing to a URL.
placeholder: Public URL of your asset

Applying

Applying this token would have an effect on the fills. When applying an asset token we need to fetch the image from the provided source and then apply that as an image fill. If both an asset and a color fill are present, the asset should win.
Related docs: https://www.figma.com/plugin-docs/working-with-images/

[jpzwarte]

So we have a slightly different use case: different icons per theme. We want to be able to have different icons in the design system component based on the theme. And use SVGs inside Figma for them (not every product uses FontAwesome).

[jpzwarte]

So in our case we would have a place in Figma where the SVG icons are defined. And then using asset tokens, we would "point" the Figma token to the icon "element" on that Figma page??? And Figma Tokens would perhaps serialise the icon inside the JSON?

[jpzwarte]

So we would also like to be able to use FontAwesome icons and custom icons in Figma. Some apps may be either FontAwesome, or all custom icons. And some may even use mostly FontAwesome, but have a few custom icons for specific ones.

[six7]

This was shipped as part of V123 🎉