[SDV 2.0] Add to_dict method for all constraints #3017

Problem Description

Now that SDMetrics accepts constraint dictionaries, it would be helpful to for constraints to have a to_dict method so that constraint dictionaries can be gotten directly without needing to go through a synthesizer.

Expected behavior

Add a to_dict method to all constraints. This method should return the dictionary representation of the constraint, like the other to_dict methods we have. Currently, constraints have the get_constraint_dict method which implements this functionality. The to_dict method can just be a wrapper around this existing method.