Support reflected event types for `Observer` triggers.

[ItsDoot]

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:

fn my_observer(trigger: Trigger<ReflectedEvent>, /* ... */) {
    /* ... */
}

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