Add getter to assetserver for loaders

## What problem does this solve or what need does it fill?

It would be nice if bevy allowed you to get a mutable reference to an asset loader after it has been added to the asset server, this would allow people to add customisable asset loaders, my specific use case is a 2d animation system that uses nodes, it would be nice if you could register animation nodes into the loader after the face at the moment I handle all the node loading outside the server and will have to come up with some workaround to allow custom nodes types to be added to the loader if I want to move it inside the asset server, the nodes are part of larger files so don't have specific file extension meaning I can't just add more loaders to the asset server.

## What solution would you like?

add a getter like fn get_asset_loader_mut(&self, extension: &str) -> Result<&mut Arc, AssetServerError>
this would allow me to add a way for users of the plugin to add their nodes.
this might need the Arc to be changed to a RWLock

## What alternative(s) have you considered?

make the current fn get_asset_loader(&self, extension: &str) -> Result, AssetServerError> pub so that it is at least possible to get the current loader to clone modify and add it back as a new loader with the same extension

## Additional context
I don't know it this is currently possible and I'm just missing it