Add benchmarks or stress tests for asset loading and processing

> We have a pseudo stress test in the form of running the asset processor on everything in the `assets` folder. This is as simple as turning on `AssetPlugin::processed_dev()` in any example and running it. It will log the total duration. To repeat, just delete the `imported_assets` folder and run again.
> I ran that about 10 times for both main and this pr before submitting it and the change here was dwarfed by very swingey results (my guess is filesystem i/o). We might need something a bit more targeted to isolate the perf difference.

_Originally posted by @cart in https://github.com/bevyengine/bevy/issues/9729#issuecomment-1712371700_