Support reflected event types for `Observer` triggers.

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

We don't currently support this as a safe alternative to #17509. Users might find this useful.

## What solution would you like?

In a similar manner to #17509, we provide a `ReflectedEvent` event type for use in a `Trigger`:

```rust
fn my_observer(trigger: Trigger, /* ... */) {
/* ... */
}
```

We would provide a type function to map `ReflectedEvent` into `&dyn Reflect` for `Trigger::event()` and `&mut dyn Reflect` for `Trigger::event_mut()`.