Feature Request: Collision Normal Velocity

[CleanCut]

From this Discord thread. (Please fix my wording if I stated the request incorrectly!)

• Add a field to SolveContact to track normal velocity.
• Make it so that you can modify solver_contact.normal_velocity in bevy_rapier. (should I make a separate issue in bevy_rapier for this?)

I'm trying to make it so that I can produce a fixed-velocity collision response in my game, to simulate powered bumpers in a pinball machine which produce a large fixed response (almost) regardless of collision velocity.