[icon] add support for loading SVG icons from arbitrary URLs

[DRiFTy17]

Describe the feature: The <forge-icon> currently has a src property/attribute that allows for providing any <svg> element as a string. This component should also support providing a custom URL to have the component attempt to load the <svg> content from that URL to avoid having developers to have to fetch the content themselves and use the src property.

Describe any alternatives you've considered: Using the existing fetchIconContent(url, name) method that is exported from the library to fetch the icon content from a URL, and then manually set the content using the src property.

Additional context: Developers need to be sure that their SVG assets can be served over CORS if fetching from a different domain.