[Viewmodel] Support flow StateObservable on iOS

[npresseault]

• Support Flow in NullableStateObservable and StateObservable. initial value must be provided.
• Support Flow in PilotAccessibility component bindings.

We had a specific use case where we wanted to use a Flow instead of a StateFlow for accessibility but I'm pretty sure we could find more use cases for Flow observation.

This is the equivalent of using collectAsState on android.

@Composable
public fun <T : R, R> Flow<T>.collectAsState(
    initial: R,
    context: CoroutineContext = EmptyCoroutineContext,
): State<R>