Add support for the defaultMapping property in the discriminator object

[baywet]

OAI 3.2.0 defines a new default mapping property for the discriminator object.

We should add this property, (type OpenAPISchemaReference), implement serialization, deserialization for it in 3.2.0.

In 3.1.0 and earlier version, any value there should be serialized to an x-oas-default-mapping extension value instead.