Make ObservedBy public (#17297)

# Objective
- Currently, the `ObservedBy`-component is only public within the
`bevy_ecs` crate. Sometimes it is desirable to refer to this component
in the "game-code". Two examples that come in mind:
- Clearing all components in an entity, but intending to keep the
existing observers: Making `ObservedBy` public allows us to use
`commands.entity(entity).retain::<ObservedBy>();`, which clears all
other components, but keeps `ObservedBy`, which prevents the Observers
from despawning.
- The opposite of the above, clearing all of entities' Observers:
`commands.entity(entity).remove::<ObservedBy>` will despawn all
associated Observers. Admittedly, a cleaner solution would be something
like `commands.entity(entity).clear_observers()`, but this is
sufficient.

## Solution

- Removed `(crate)` "rule" and added `ObservedBy` to the prelude-module

## Testing

- Linked `bevy_ecs` locally with another project to see if `ObservedBy`
could be referenced.

crates/bevy_ecs/src/observer/entity_observer.rs

@@ -8,7 +8,7 @@ use alloc::vec::Vec;
 
 /// Tracks a list of entity observers for the [`Entity`] [`ObservedBy`] is added to.
 #[derive(Default)]
-pub(crate) struct ObservedBy(pub(crate) Vec<Entity>);
+pub struct ObservedBy(pub(crate) Vec<Entity>);
 
 impl Component for ObservedBy {
     const STORAGE_TYPE: StorageType = StorageType::SparseSet;

crates/bevy_ecs/src/observer/mod.rs

@@ -3,14 +3,13 @@
 mod entity_observer;
 mod runner;
 
-pub use entity_observer::CloneEntityWithObserversExt;
+pub use entity_observer::{CloneEntityWithObserversExt, ObservedBy};
 pub use runner::*;
 
 use crate::{
     archetype::ArchetypeFlags,
     component::ComponentId,
     entity::EntityHashMap,
-    observer::entity_observer::ObservedBy,
     prelude::*,
     system::IntoObserverSystem,
     world::{DeferredWorld, *},