Add debug rendering axes and joints and improve configuration API

[Jondolf]

Adds several methods for PhysicsDebugConfig and DebugRender to make the API much nicer, and adds support for debug rendering entity axes and joints.

An example of a global config that renders colliders and hides the visual meshes, but disables all debug rendering for a specific entity:

app.insert_resource(
    PhysicsDebugConfig::colliders(Color::ORANGE).with_mesh_visibility(false)
);

// In some setup system
commands.spawn((
    RigidBody::Dynamic,
    Collider::ball(0.5),
    DebugRender::none(),
));

Closes #75 (although it was already possible, just with a more cumbersome API)