Joints as entities

The current API for (impulse) joints is not idiomatic to ECS:

A body contains its joint data, and contains a reference to its other body. This leads to the following surprises:
- how to choose which body has the joint data ?
- we cannot create 2 joints on a same entity, and have to leverage hierarchy to represent multiple impulse joints.
- API to query joint data is difficult to make ergonomic (see https://github.com/dimforge/bevy_rapier/pull/530), because data is split between:
- body1: `Entity` or other component `RigidbodyHandle`
- body2: `parent` within the component `ImpulseJoint`.

## Suggested change

I suggest moving the `ImpulseJoint` out of the body, to have a separate entity containing all information: joint data + the linked entities.

This way we could model multiple joints per body and have all joint data available in the same fashion.

To be noted this is also the choice of Avian (see https://docs.rs/avian3d/latest/avian3d/dynamics/solver/joints/struct.FixedJoint.html)