Make the ObservedBy component useful to public consumers (#19591)

# Objective

As raised by @Jondolf, this type is `pub`, and useful for various
consumers to ensure cleanup or debugging.

However, it doesn't offer any way to actually view the data.

## Solution

- Add a read-only view of the data.
- Don't add any (easy) way to mutate the data, as this presents a huge
footgun.
- Implement Reflect and register the component so you can see it in
inspectors nicely.

crates/bevy_app/src/app.rs

@@ -106,10 +106,13 @@ impl Default for App {
 
         #[cfg(feature = "bevy_reflect")]
         {
+            use bevy_ecs::observer::ObservedBy;
+
             app.init_resource::<AppTypeRegistry>();
             app.register_type::<Name>();
             app.register_type::<ChildOf>();
             app.register_type::<Children>();
+            app.register_type::<ObservedBy>();
         }
 
         #[cfg(feature = "reflect_functions")]

crates/bevy_ecs/src/observer/entity_observer.rs

@@ -6,12 +6,24 @@ use crate::{
 };
 use alloc::vec::Vec;
 
+#[cfg(feature = "bevy_reflect")]
+use crate::prelude::ReflectComponent;
+
 use super::Observer;
 
 /// Tracks a list of entity observers for the [`Entity`] [`ObservedBy`] is added to.
-#[derive(Default)]
+#[derive(Default, Debug)]
+#[cfg_attr(feature = "bevy_reflect", derive(bevy_reflect::Reflect))]
+#[cfg_attr(feature = "bevy_reflect", reflect(Component, Debug))]
 pub struct ObservedBy(pub(crate) Vec<Entity>);
 
+impl ObservedBy {
+    /// Provides a read-only reference to the list of entities observing this entity.
+    pub fn get(&self) -> &[Entity] {
+        &self.0
+    }
+}
+
 impl Component for ObservedBy {
     const STORAGE_TYPE: StorageType = StorageType::SparseSet;
     type Mutability = Mutable;