Add ColliderBackendPlugin and support generic colliders

[Jondolf]

Objective

An improved version of #302.

Currently, collider logic is split across several plugins. The PreparePlugin initializes them and manages mass properties and collider parents, the SyncPlugin handles collider scale and transform propagation, the BroadPhasePlugin updates their AABBs and collects potential collision pairs, and the NarrowPhasePlugin uses them for actually computing contacts.

Using Collider in so many places isn't ideal since it couples collision logic with other plugins, and it also makes it very challenging to implement custom collision backends nicely. It would be useful to handle collider-specific background work in a single plugin and to support custom colliders through generics.

Solution

• Add a ColliderBackendPlugin. It handles collider initialization, transform propagation, scaling, parents, mass property updates, AABB updates, and more.
• Add the AnyCollider and ScalableCollider traits. Implementing these for a component makes it possible to use a completely custom collider component by passing the type to ColliderBackendPlugin and NarrowPhasePlugin through generics. There is a new custom_collider example that demonstrates this.

To avoid leaking too many implementation details related to system ordering, I also added more system sets like the BroadPhaseSet, NarrowPhaseSet and SyncSet enums.

Creating a custom collision backend is now straightforward. After implementing AnyCollider and ScalableCollider for a component, simply add the CollisionBackendPlugin and NarrowPhasePlugin with that type:

.add_plugins((
    DefaultPlugins,
    PhysicsPlugins::default(),
    ColliderBackendPlugin::<CircleCollider>::default(),
    NarrowPhasePlugin::<CircleCollider>::default(),
))

---

Changelog

Added

• CollisionBackendPlugin
• AnyCollider trait
• ScalableCollider trait
• scale_by method for colliders
• Collider::swept_aabb
• ColliderAabb::new
• ColliderAabb::merged
• BroadPhaseSet system sets
• NarrowPhaseSet system sets
• SyncSet system sets
• custom_collider example

Changed

• Moved collider module from components to collision module
• NarrowPhasePlugin takes a generic type implementing AnyCollider
• Moved collider systems from PreparePlugin, SyncPlugin and SleepingPlugin to ColliderBackendPlugin
• Collider logic from upate_mass_properties is in a new update_collider_mass_properties system

Migration Guide

• Replace NarrowPhasePlugin with NarrowPhasePlugin::<Collider> if you add it manually
• Replace Collider::compute_aabb with Collider::aabb