Support vectors of dynamic assets in dynamic asset files

[NiklasEi]

This allows for dynamic asset files like

({
    "files": [
        File(path: "images/tree.png"),
        File(path: "images/player.png"),
    ],
})

and even works for custom dynamic assets and nested collections

({
    "images": [
        CombinedImage (
            bottom_layer: "images/tree.png",
            top_layer: "images/player.png",
        ),
        [
            File(path: "images/tree.png"),
            File(path: "images/player.png"),
        ],
        Folder(path: "images")
    ],
})

ToDo

• Update bevy_common_assets when the new ron version is published Upcoming v0.9 release ron-rs/ron#496
• Simplify/double-check trait bounds
• Remove StandardDynamicAsset::Files in preference of defining vecs in the asset files?
• Make OneOrManyDynamicAssets optional
  • Maybe move the asset wrapper type to a trait as an associated type?
  • Write a test to ensure users can add custom dynamic assets without it
• Introduce a new feature for dynamic assets to hide serde behind it?
  • Check if bevy_asset doesn't already require serde (Yes, it does)

Resolves #78