Use a C++ callback instead of Callable for synchronizing physics nodes' state with physics servers.
Remove usage of PhysicsDirectBodyState in physics nodes when not
necessary.
Store PhysicsDirectBodyState for bodies individually instead of a
singleton to avoid issues when accessing direct body state for multiple
bodies.
PhysicsDirectBodyState is initialized only when needed, so it doesn't
have to be created when using the physics server directly.
Move PhysicsDirectBodyState2D and PhysicsDirectBodyState3D to separate
cpp files.
