add support for the mediaType itemSchema property

[baywet]

OAI 3.2.0 defines a new itemSchema property on the media type object.

This property is of type IOpenAPISchema? and serialization/deserialization should be updates to account for this new field.

When serializing to version 3.1.0, the property name should be prefixed with x-oai- so it's treated as an extension.