Option for ShapeCasts to return only face normals

A limitation with Parry I've encountered while using Rapier3d to implement a character controller. Sometimes shapecasts will return the normal of a vertex or edge feature when a face normal is necessary for things like wall/ramp sliding logic.

Not sure what the best way to go about implementing this feature would be, somebody in the bevy physics channel suggested adding a `force_face_normals` field to ShapeCastOptions.

Possible related issue: https://github.com/dimforge/parry/issues/193