Sync Point Event

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

Wanting to do something at every sync point, and Events seem like the easiest option to make that available.

## What solution would you like?

```rust
fn observer(_: On) {
//Do stuff per sync point
}
```

Could potentially be PreSync and PostSync, for before and after commands have been applied, although the naive/single approach before commands is probably fine.

## Additional context

Prior PR: [#19818](https://github.com/bevyengine/bevy/pull/19818)

Start by reading prior PR #19818 and tracing the sync point where commands are applied. Decide whether the event is emitted before application or needs PreSync and PostSync variants; done means observers can reliably run at every sync point with the intended timing.

Written by the indexing model from the issue text.