add a way of getting the "impact normal" from a shape cast hit.

[atornity]

The hit result struct in unreal engine has this thing called ImpactNormal which is the furhest (from the sweep direction) face normal on the hit collider. When you hit a corner (on a cube for example) and the cast shape is a sphere, then the hit normal will point towards the sphere, but sometimes I want the normal of the closest face.

In particular, I need this for floor detection and stair stepping for a character controller I'm working on.