LowLevel 2D Physics Object Callback Targets use MonoBehaviour rather than System.Object

Callback targets currently restricted to using the `MonoBehaviour` type however this should in-fact be `System.Object` which doesn't artificially restrict use-cases to user scripts.

See: https://docs.unity3d.com/6000.3/Documentation/ScriptReference/30_search.html?q=callbacktarget

Discussions: https://discussions.unity.com/t/is-it-possible-to-register-low-level-2d-physics-callbacks-without-monobehaviour/1688725