wasm asset listing from s3

[varoonp123]

The only thing @cdsupina needs to use is

/// A collection of file names/paths for individual background image assets to be lazily loaded.
/// These are fit to be used with bevy's AssetServer::load. Many architectures will support
/// bevy::asset::AssetIO::read_directory . But wasm32 does not support it. So we have a fallback
/// that can be populated by other systems. Generally this will be written to once and never
/// updated again.
#[derive(Resource, Deref, DerefMut, Default, From)]
pub struct BackupBackgroundAssetPaths(pub Vec<String>);