More safely support untyped Observer events

[ItsDoot]

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

We don't currently support specifying an untyped event as the event type in an Observer Trigger. Our "support" for it currently involves specifying some rust type as the event type and providing a Trigger::event_ptr function which returns a Ptr, also not providing a version that returns PtrMut.
Doing it this way can cause accidental UB on the user side very easily if they don't specify a type compatible with all of the event types.

What solution would you like?

We should directly support specifying untyped events as the event type in the Trigger like so:

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

And then we would have a type function that maps UntypedEvent into a Ptr for Trigger::event() and a PtrMut for Trigger::event_mut().

Additional context

Originally attempted in #14674 but that PR languished due to a too large of a scope.