Expose ObserverDescriptor fields (#17623)

# Objective

Expose accessor functions to the `ObserverDescriptor`, so that users can
use the `Observer` component to inspect what the observer is watching.
This would be useful for me, I don't think there's any reason to hide
these.

crates/bevy_ecs/src/observer/mod.rs

@@ -307,6 +307,21 @@ impl ObserverDescriptor {
             .extend(descriptor.components.iter().copied());
         self.entities.extend(descriptor.entities.iter().copied());
     }
+
+    /// Returns the `events` that the observer is watching.
+    pub fn events(&self) -> &[ComponentId] {
+        &self.events
+    }
+
+    /// Returns the `components` that the observer is watching.
+    pub fn components(&self) -> &[ComponentId] {
+        &self.components
+    }
+
+    /// Returns the `entities` that the observer is watching.
+    pub fn entities(&self) -> &[Entity] {
+        &self.entities
+    }
 }
 
 /// Event trigger metadata for a given [`Observer`],

crates/bevy_ecs/src/observer/runner.rs

@@ -312,6 +312,11 @@ impl Observer {
         self.descriptor.events.push(event);
         self
     }
+
+    /// Returns the [`ObserverDescriptor`] for this [`Observer`].
+    pub fn descriptor(&self) -> &ObserverDescriptor {
+        &self.descriptor
+    }
 }
 
 impl Component for Observer {