add HarnessState to callbacks, move HarnessPlugin to src_testbed/harness/plugin

src_testbed/harness/plugin.rs

@@ -0,0 +1,10 @@
+use crate::harness::HarnessState;
+use crate::PhysicsState;
+use crate::physics::PhysicsEvents;
+
+pub trait HarnessPlugin {
+    //FIXME: is run_callbacks needed?
+    fn run_callbacks(&mut self, physics: &mut PhysicsState, events: &PhysicsEvents, harness_state: &HarnessState, t: f32);
+    fn step(&mut self, physics: &mut PhysicsState);
+    fn profiling_string(&self) -> String;
+}