move code out of "shared" and into new "physics" lib

If we're going to try out Bullet as a physics engine we want to put it behind a thin translation layer.  This is mainly to reduce leakage of Bullet specific classed into application code (Bullet has its own Vector3 class) but also to make it easier to upgrade Bullet in the future, or even to try out a different physics engine entirely should we want to.

To that end I'm splitting our existing physics code out of the "shared" lib and putting it into its own library.
I stubbed out some Bullet-discovery cmake logic here, but it is commented out because we're not actually using Bullet yet.