Expose more functionality of RenderVisibleEntitiesClass as public. (#24180)

# Objective

Encountered while porting the bevy_mod_outline ecosystem crate to Bevy
_0.19_:

The outline effects have a different visual extent to the geometry of
the meshes they decorate. Hence, I need to manage outline visibility
separate from Bevy's own visibility system. I would like to use
`DirtySpecialization` in my custom rendering phase as suggested by the
draft migration guide, but its API depends on supplying a
`RenderVisibleEntitiesClass`. The methods (and one field) of this type
are non-public so my own visibility system cannot manufacture one.

## Solution

- Expose added_entities, prepare_for_new_frame(), and
update_cpu_culled_entities() as public.

## Testing

- Check Bevy builds and members are accessible from a dependent crate.