Clamp numerical properties on USD prims to valid values

[corybarr]

Unreal's editor UI offers some clamping abilities. For example, several settings are clamped to 0:

  UPROPERTY(
      EditAnywhere,
      BlueprintReadWrite,
      Category = "Cesium",
      meta = (EditCondition = "bSpecifyZoomLevels", ClampMin = 0))
  int32 MinimumLevel = 0;

The Omniverse extension could benefit from this UI-level clamping. We can currently do this via sliders, but clamping should be possible even when a slider doesn't make sense.