Add support for joints and motors

[AyeGill]

Well, the title explains it: it would be nice to support joints and motors.

bevy_rapier implements these as separate entities. For example, one does:

let joint = BallJoint::new(Vec3::new(0.0, 0.0, 1.0).into(), Vec3::new(0.0, 0.0, -3.0).into());
commands.spawn().insert(JointBuilderComponent::new(joint, entity1, entity2));

This seems like the most reasonable way of doing things. It is

My suggestion would be to have something like a "Joint" struct, containing the data necessary to construct a joint, including two entities. Then the "Joint system" could go over all the changed joint components each step, creating/modifying/deleting joints from the JointSet as needed. The motor configuration could either be a separate component or an optional part of the joint component (the advantage of doing it the second way is that the type of a motor configuration depends on the joint type - so this allows us to prevent people from adding incompatible motor and joints to the same entity).

I'm imagining using this something like

commands.spawn().insert(Joint::Ball(entity_1, entity_2, Vec3::new(1.0,0.0,0.0), Vec3::new(-1.0,0.0,0.0)))

I'm happy to take a crack at implementing this myself if this sounds like a reasonable solution.

[jcornaz]

Sounds good to me. Thanks for the suggestion and for taking a look at it :-)