feat: Add basic lifecycle instrumentation.

[kinyoklion]

Summary

Adds basic support for lifecycle instrumentation.

The code inside the instrumentation/lifecycle/platform folder is a port of the code in the LD flutter SDK. Potentially we could move that to a shared library. We split the LD SDK into layers, but there wasn't any need for a flutter specific platform layer sans SDK. (It is dart only, then client-side dart, and then flutter. But no flutter platform library.)

How did you test this change?

Manual testing.

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Note

Introduce lifecycle instrumentation that emits spans on Flutter app state changes across IO and Web platforms, and wire it into the observability plugin.

• Observability:
  • Lifecycle instrumentation:
    • Add Instrumentation interface and LifecycleInstrumentation that listens to AppLifecycleState and emits spans via Observe with attributes from LifecycleConventions.
    • Define LifecycleConventions with span name device.app.lifecycle and attribute flutter.app.state mapped from AppLifecycleState.
    • Implement platform listeners platform/io_lifecycle_listener.dart (uses AppLifecycleListener) and platform/js_lifecycle_listener.dart (uses document visibility); include a stub fallback.
  • Plugin integration:
    • Update ObservabilityPlugin to instantiate and hold LifecycleInstrumentation on construction.

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