Include ObjectID for create_test_image_2d

[JHancox]

Is your feature request related to a problem? Please describe. create_test_image_2d is great, but at the moment, you can get the class ID, but not the object instance ID. This means that two overlapping objects effectively become one large object (i.e. the boundary is undefined).

Describe the solution you'd like It would be really useful if the output could include a channel for each object ID. This would allow individual objects to be discerned

Describe alternatives you've considered For now, I have created my own class to accomplish this, but would prefer to use the Monai Core version

Additional context This is a useful feature for testing loss when it comes to nucleus instance segmentation in histopathology, where there are frequently overlapping nuclei.