Fix AssetTransformer breaking LabeledAssets

[RyanSpaker]

Objective

• AssetTransformer provides an input asset, and output an asset, but provides no access to the LabeledAsset's created by the AssetLoader. Labeled sub assets are an extremely important piece of many assets, Gltf in particular, and without them the amount of transformation on an asset is limited. In order for AssetTransformer's to be useful, they need to have access to these sub assets.
• LabeledAsset's loaded by AssetLoaders are provided to AssetSavers in the LoadAndSave process, but the LoadTransformAndSave process drops these values in the transform stage, and so AssetSaver is given none.
• Fixes LoadTransformAndSave Drops Asset Dependencies #11606

Ideally the AssetTransformer should not ignore labeled sub assets, and they should be kept at least for the AssetSaver

Solution

• I created a new struct similar to SavedAsset named TransformedAsset which holds the input asset, and the HashMap of LabeledAssets. The transform function now takes as input a TransformedAsset, and returns a TransformedAsset::<AssetOutput>. This gives the transform function access to the labeled sub assets created by the AssetLoader.
• I also created TransformedSubAsset which holds mutable references to a sub asset and that sub assets HashMap of LabeledAssets. This allows you to travers the Tree of LabeledAssets by reference relatively easily.
• The LoadTransformAndSave processor was then reworked to use the new structs, stopping the LabeledAssets from being dropped.

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Changelog

• Created TransformedAsset struct and TransformedSubAsset struct.
• Changed get_untyped_handle to return a UntypedHandle directly rather than a reference and added get_handle as a typed variant in SavedAsset and TransformedAsset
• Added SavedAsset::from_transformed as a constructor from a TransformedAsset
• Switched LoadTransformAndSave process code to work with new TransformedAsset type
• Added a ProcessError for AssetTransformer in process.rs
• Switched AssetTransformer::transform to use TransformedAsset as input and output.
• Switched AssetTransformer to use a BoxedFuture like AssetLoader and AssetSaver to allow for async transformation code.
• Updated AssetTransformer example to use new structure.

[alice-i-cecile]

@thepackett, could I get your review?

[thepackett]

I completely overlooked labeled assets when I initially implemented asset transformers. Thanks for spotting and fixing this!

[alice-i-cecile]

Thank you very much for catching and unbreaking this. I'd still like more examples in the assets folder, both for learning and testing, but that can wait for follow-up PRs.