Use hooks for component initialization

[Jondolf]

Objective

Fixes #475.

Missing components for rigid bodies, colliders, etc. are currently added at specific points in the physics shedule. This means that the world is momentarily in an invalid state before those systems are run, which can even lead to confusing crashes when using methods like query.single() for queries that expect those components to exist.

Solution

Use component hooks for initializing missing components with minimal delay. This also meaningfully simplifies scheduling.

When Bevy gets required components (Soon™), those could be used instead.

Performance

Using hooks appears to be as fast if not faster than the old initialization systems.

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Migration Guide

PrepareSet::PreInit has been renamed to PrepareSet::First, and PrepareSet::InitRigidBodies, PrepareSet::InitColliders, and PrepareSet::InitMassProperties have been removed. Most missing components are now initialized by component lifecycle hooks.

CcdPlugin and SpatialQueryPipeline no longer store a schedule and are now unit structs. Instead of SpatialQueryPlugin::new(my_schedule) or SpatialQueryPlugin::default(), just use SpatialQueryPlugin.