Custom AssetSource(s) must be registered before DefaultPlugins in order to work

## Bevy version

0.12-dev

## What you did

I attempted to register an AssetSource:

```rust
fn main() {
App::new()
.add_plugins(
DefaultPlugins.set(AssetPlugin {
mode: AssetMode::ProcessedDev,
..default()
}),
)
.register_asset_source(
"test",
AssetSource::build()
.with_reader(|| Box::new(FileAssetReader::new("test")))
)
.run();
}
```

## What went wrong

When I tried to load a handle from that AssetSource, I got this error:
```rust
ERROR bevy_asset::server: Asset Source 'AssetSourceId::Name(test)' does not exist
```

## Additional information

Registering the asset source _before_ adding the `DefaultPlugins` resolved the issue - this seems a bit counter-intuitive though.

I think there's broadly 2 options to stop others encountering this issue:

1) Mention this ordering requirement in the `register_asset_source` comments:
https://github.com/bevyengine/bevy/blob/6b070b1776c63aa69cc60185e04c293b32e60164/crates/bevy_asset/src/lib.rs#L244-L245

2) Call `build_sources` from `Plugin::finish()` instead of `Plugin::build()`
https://github.com/bevyengine/bevy/blob/6b070b1776c63aa69cc60185e04c293b32e60164/crates/bevy_asset/src/lib.rs#L139-L140
This might not work for anyone attempting to load handles from a custom `AssetSource` in a `FromWorld` impl, although a similar issue was resolved in https://github.com/bevyengine/bevy/pull/9429

Start in crates/bevy_asset/src/lib.rs, especially the register_asset_source comments and the build_sources call in Plugin::build(). Reproduce the reported ordering with DefaultPlugins and a custom FileAssetReader, then determine whether the ordering should be documented or the source-building lifecycle changed. Done means a custom AssetSource works as expected or the ordering requirement is clearly documented, with coverage for the reported case.

Written by the indexing model from the issue text.