Enable dynamic mutable access to component data

[willcrichton]

I'm working on developing a world visualizer/editor for Bevy. See a demo: https://discord.com/channels/691052431525675048/692648638823923732/801608361463513089

To work, the tool needs to get mutable access to every component in the world, as well as find a method for displaying the component in a UI. Right now, I'm using this strategy:

1. Maintain a mapping from TypeId to pre-registered types that implement the Inspectable trait in bevy-inspector-egui (see here). For these types, I use archetype.get_dynamic to get the component data as *mut u8 then transmute it to the appropriate type.
2. If Inspectable is not implemented, then try to cast the component to &mut dyn Reflect using ReflectComponent, then use bevy-inspector-egui to generate a generic UI for components implementing Reflect.

This PR adds two changes to make this possible. First, it publicly exposes Archetype::get_dynamic. Second, it adds a new method ReflectComponent::reflect_component_mut that returns &mut dyn Reflect instead of just &dyn Reflect.

Note: obviously the proliferation of unsafe code and transmute is concerning here. If there's a different approach that works without these changes, I'd be happy to hear.

[bjorn3]

You should probably squash.

[cart]

Its good to call out the "scheduler" side of things in the "Safety" section, but we should also call out generically that this could violate rust's mutability rules. Callers should verify that they are not violating the rules.