gpui: Defining multiple AssetSources

[Bloeckchengrafik]

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Describe the feature

Trying to build a gpui-library for using lucide-icons, I noticed that there is no way of providing assets without adding some kind of hook into the user's AssetSource. I think this is not optimal behaviour. I don't know yet, how this could be implemented specifically.

If applicable, add mockups / screenshots to help present your vision of the feature

No response