Detect when all assets are loaded

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

In Bevy's examples a lot of work is required to manually track assets has finished loading:
- https://bevy.org/examples/assets/multi-asset-sync/
- https://bevy.org/examples/games/loading-screen/

I'm also working on a [screenshot testing framework](https://github.com/azureblaze/bevy_mod_screenshot_test) which ideally should let users wait for all members of the scene to be spawned in before a screenshot is taken.

While screenshot tests are more niche and can afford to do hacky things like waiting for fixed time, the current way to implement a loading screen seems very complicated and hard to scale.

## What solution would you like?

Some way to tell when the AssetServer/other related systems are not doing anything anymore.

## What alternative(s) have you considered?

require users to place asset barriers everywhere

Start by reading AssetServer and the related asset-loading systems, then compare the multi-asset-sync and loading-screen examples linked in the issue. Determine how the engine could expose an idle state without requiring asset barriers; done means users can reliably tell when the relevant assets and scene members are ready.

Written by the indexing model from the issue text.