gpui: Add SVG rendering to `img` element and generic asset cache (#9931)

This is a follow up to #9436 . It has a cleaner API and generalized the
image_cache to be a generic asset cache, that all GPUI elements can make
use off. The changes have been discussed with @mikayla-maki on Discord.

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Co-authored-by: Mikayla <mikayla@zed.dev>