Feature Request: Collision Normal Velocity

_From [this Discord thread](https://discord.com/channels/507548572338880513/748627261384556715/1044454804908412948). (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.