Fix 11208: Scale relative to the position of the interactors, not the attach points

[shaynie]

Overview

Change ScaleLogic.cs to scale relative to the distance between two interactors instead of the distance between their attach points.

Changes

• Fixes: Two hand object manipulation of type scaling not scaling the right way in MRTK3 #11208.

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[thalbern]

consider making this configurable to support both scenarios :)