Event triggering

Explores an API for triggering events and querying them at runtime.

For Flutter we expose an onEvent callback that you can add a callback to on the StateMachineController:
```dart
final controller = StateMachineController.fromArtboard(artboard, 'bumpy', onEvent: {);
controller.onEvent = (event) {
  // Do something with event. Like:
  if(event is OpenURLEvent) {
    launchUrl(event.url);
  }
};
artboard.addController(controller!);
```

Note that I haven't piped onEvent to the Flutter runtime yet but you'll see it in the StateMachineController in core.

In C++:
```c++
auto count = stateMachineInstance->firedEventCount();
for(auto i = 0; i < count; i++) {
  auto event = stateMachineInstance->firedEventAt(i);
  if(event->is<OpenURLEvent>()) {
    // Do something with the url.
    auto url = event->as<OpenURLEvent>()->url();
  }
}
```

You can see some of this in action in the state_machine_event_test.cpp.

You can also see the events in the console in the editor:
<img width="717" alt="CleanShot 2023-08-10 at 18 03 22@2x" src="https://github.com/rive-app/rive/assets/454182/af21902a-424d-435b-b5b0-2a43701fe186">

In a follow up PR:
- Ability to trigger events from State (in/out) and Transition (start/end).
- Add custom properties to Events C++ API (currently they are loaded but not tracked against their respective events).

Diffs=
8caa7d377 Event triggering (#5793)

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