No convenient way to set default assets through the `AssetServer`

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

Currently, it seems like the simplest way to set the default value of an `Assets` collection is this:

```rust
fn load_default_font(mut fonts: ResMut>) {
let bytes = include_bytes!("font.ttf");
let font = Font::try_from_bytes(bytes.to_vec()).unwrap();

fonts.insert(AssetId::::DEFAULT_UUID, font);
}
```

This sidesteps the whole asset system, and requires including potentially large data in the binary (in my case, a 14 MiB font file), so it's not ideal.

`AssetServer` has no methods that load an asset into an existing `Handle`/`AssetId`, so it cannot directly be used for this. It's possible to load the asset with a different `Handle`, listen to the asset event that indicates it has loaded, and then copy the asset over to the default ID. This is extremely inconvenient, though.

## What solution would you like?

`AssetServer` should expose some way of asynchronously loading an asset into an existing `Handle` or `AssetId`, overwriting whatever is already there.

## What alternative(s) have you considered?

None so far. Open to suggestions.