Large crate rework using component hooks and task pools

[vertesians]

Uh, where do I begin...

• There's now a new type of Asset called a Billboard. Billboards are created from a Handle<Imge> (and optionally a Handle<TextureAtlasLayout>) and handle creating the mesh and other tasks needed to prepare a 3D sprite. A Handle<Billboard> is associated with an entity by inserting the Sprite3dBillboard component.
• Sprite3dBuilder, Sprite3dParams, and Sprite3dBundle have been removed.
• The crate no longer handles caching, users should clone Handle<Billboard> instead.
• It's no longer necessary for users to check and wait for images to load before spawning 3d sprites, the crate handles all the details internally.
• Previously, Sprite3dBundle would include defaults for StandardMaterial that worked well with 3d sprites. Since the bundle was removed, the utility function bevy_sprite3d::utils::material was added to provide a convenient source for these defaults. Sprite3dBillboard inserts a preconfigured MeshMaterial3d if not overriden, so you usually don't need to use it.

I don't know if this PR will be accepted since it changes so much, but might as well put it out there.

[FraserLee]

Thanks for the work on this. I'm pretty busy at the moment, but @beaumccartney should be able to take a look at this one.