[ReflectionBasedXmlSerializer] To Support Serializing XmlSchemaObject

[shmao]

XmlSchemaObject is currently not in the contract. If we add it into the contract, we need to work on this issue.

[shmao]

XmlSchemaObject is available in Net Standard 2.0. We should consider to make reflection based serialization to support the serialization of the type.

[StephenMolloy]

I think this was closed by mistake? There is no comment here, and the NYI exception still exists in the code and it looks to me like we can de/serialize XmlSchema with the ILGen serializer, but run into the NYI when running with the reflection-based serializer.