`ColliderCachePlugin` is missing a dependency on `AssetEvent<Mesh>` / `bevy_mesh::MeshPlugin`

[mgi388]

avian3d: 0.6.

In Bevy's MeshPlugin it calls app.init_asset::<Mesh>().

But Avian never registers MeshPlugin itself.

This means that if you are, say, writing a user land unit test which installs Avian's PhysicsPlugins::default() (along with, say, MinimalPlugins and the user land plugin you are testing), then when the unit test runs, it will crash with:

thread 'tests::test_my test' (141545) panicked at /me/bevy/crates/bevy_ecs/src/error/handler.rs:125:1:
Encountered an error in system `avian3d::collision::collider::cache::clear_unused_colliders`: Parameter `MessageReader<'_, '_, AssetEvent<Mesh>>::messages` failed validation: Message not initialized
If this is an expected state, wrap the parameter in `Option<T>` and handle `None` when it happens, or wrap the parameter in `If<T>` to skip the system when it happens.

The options I see are:

1. Add If to the system param.
2. In ColliderCachePlugin, make it conditionally add MeshPlugin since it necessarily depends on it being registered.