Add Unmangled Discriminator Property Name to Discriminator Class

The Discriminator class contains some information to inform how deserialization should occur, given the property name and the mapping to subclasses I would expect to be able to dispatch appropriately.

Unfortunately the propertyName property is mangled before it is passed to the template generation, so for example in the python client generator, a property of petSpecies is only available as pet_species. If you then want to use this property to fetch data from the API response, you need to guess what the original form of the property name was.

It would be very helpful if the Discriminator class also stored the original, unmangled, property name. This would make the class actually useful for the purpose of dispatching to subclasses whilst deserializing responses.

Start with the Discriminator class and trace where its propertyName is transformed before template generation, particularly along the Python client generator path described in the issue. Determine how the original property name can be retained alongside the mangled value. Done means the Discriminator exposes the unmangled name for deserialization dispatch without losing the existing mapped value.