Support sum ADTs with discriminator

[jakipatryk]

https://swagger.io/docs/specification/data-models/inheritance-and-polymorphism/

It should be possible to configure the behaviour:

• either use discriminator with sum ADTs or not
• discriminator property name: by default <sealed-type-name>Type, but with possible customization function in config
• hen sum ADT is nested, it should be configurable whether child ADTs should have both parent and its own discriminators or just their own discriminators