Add Custom Icon Capability

[rmartine-ias]

Other result list items, e.g. TechDocs have the ability to pass custom icons. This patch adds the same functionality to this plugin. My TypeScript isn't great, so please let me know if there's anything you want me to change :)

This patch lets us have icons that match in theme by specifying a custom icon:

This change also aligns the styles used with upstream Backstage, so the icons line up: