Add various methods to get reflected components from EntityRef & friends

[SkiFire13]

Objective

• feat: Add World::get_reflect() and World::get_reflect_mut() #14416 added World::get_reflect and World::get_reflect_mut, but they require a world reference. Make them available on EntityRef and other similar types.
• Also add missing _ref variants of get_by_id methods and the above get_reflect methods.

Solution

• Add get_reflect, get_reflect_ref, get_reflect_mut and get_many_reflect_mut methods to the types UnsafeEntityCell, EntityRef, EntityMut, EntityWorldMut, FilteredEntityRef and FilteredEntityMut (the _mut variants are missing for types that don't provide mutable access);
  • With the exception of EntityWorldMut, all these methods take a &TypeRegistry argument, since they can't fetch it from the World. EntityWorldMut instead fetches the AppTypeRegistry resource from the World reference it has access to;
  • Differently from the methods on World these methods take a ComponentId, which I think is more general and slightly more efficient.
• Add get_ref_by_id to those same types and a RefUntyped type to match MutUntyped.

TODO

• Remove World::get_reflect and World::get_reflect_mut, since they can be replaced by the corresponding methods on EntityRef and EntityMut/EntityWorldMut
• Documentation and safety comments
• Proper errors instead of Options
• Tests
• Showcase

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Showcase

TODO

[SkiFire13]

Closing because I thought I had enough time to finish it but lately I've been too busy to do so. Might come back to it in the future, but for now this is open to adoption if anyone wants to.