Implement Asset for Persistent<T>

[sgodwincs]

I use bevy_asset_loader for loading assets when entering a loading state. I wanted to load something using this crate as well and still get the same state management (progress/error/etc.) features provided by bevy_asset_loader.

In order to use it with this crate, Persistent<T> would need to implement Asset and have a custom AssetLoader along with registering them in the plugin. For bevy_asset_loader, it would also require custom DynamicAsset but that doesn't technically require any change in this crate (unless you'd want to support bevy_asset_loader behind a feature flag).

Two additional point:

• Would have to access it via Res(Mut)<Assets<Persistent<T>>> instead of Res(Mut)<Persistent<T>> (only if loaded via AssetLoader, the existing codepath could keep it as the latter).
• The AssetLoader::load returns a Future but this crate does synchronous loading. I'm assuming the work could be done on one of Bevy's task pools to avoid blocking. Or this crate could expose an async loading interface (maybe just internally for the AssetLoader).

Wanted to gauge whether this is something you'd want supported by this crate. If so, I'd be willing to send a PR out to do so. Let me know if I've missed anything, thanks!

[umut-sahin]

Thanks for the suggestion, it seems like a really cool feature!

I'd love to support it but I don't have much time at the moment. If you want to contribute, I'd be more than happy to help and review once it's ready. Otherwise, let's keep the issue open and I'll come back to it in a few weeks.

Thanks!