Support multiple different AssetLoaders with the same extension

[skairunner]

Currently, the AssetServer stores its references to handlers as a dictionary of string to box. But the file extension to asset type relationship is not one-to-one -- with GLTF, the file can contain anything from Meshes and Materials to Textures to Skins (aka bones).

In a language with reflection, it would be possible to select handlers depending on the type. While Rust doesn't have true reflection, it does have TypeId which would allow us to match on both AssetLoader type and file extension. This might be a good solution that would also let us preserve the existing API. This could also solve the TODO in the asset server file that mentions that load() needs to be strongly typed.

Another solution that is slightly less ergonomic is to create a load_a method that takes a String specifying the type of the asset we want to load as well as the path.

If we can determine a good path to take, I may be able to implement it, since I am working on something that directly requires this support. (No extra credit for guessing that it's GLTF :D )

[blaind]

What about allowing multiple loaders, where additional loaders for the extension could use set_labeled_asset instead of set_default_asset?

impl AssetLoader for FontLoader {
    fn load<'a>(
        &'a self,
        bytes: &'a [u8],
        load_context: &'a mut LoadContext,
    ) -> BoxedFuture<'a, Result<()>> {
        Box::pin(async move {
            load_context.set_labeled_asset("mesh", LoadedAsset::new(...));
            Ok(())
        })
    }

    fn extensions(&self) -> &[&str] {
        &["ttf"]
    }
}

Although, then the user needs to know the label when loading

let asset = asset_server.load("fonts/FiraMono-Medium.ttf#mesh");