Add generalized caching layer for Assets

[jakemac53]

I think this should just be implemented as a general CachedAssetReader which takes an instance of AssetReader, and delegates to that whenever there isn't a cached asset available. There would also be a CachedAssetWriter which takes an instance of AssetWriter and a CachedAssetReader (so it can populate the readers cache with new values).

Another option is to just implement an AssetCache class which gets passed into individual implementations of AssetReader/AssetWriter, but then each of those implementations needs to know about the cache.

[jakemac53]

Or maybe a combo, so CachedAssetReader and CachedAssetWriter each take an AssetCache.

If AssetCaches were composable this would also allow you to have a cache per phase which might be interesting.