details located in #648 and associated PR.

To deserialize to the right type we need to:

• implement generating the discriminator static method for models (reads a property in the payload and instantiates the right type)
• pass a reference to that method to the request adapter in the request executor method.
• forward the reference to that method to the parse node in the request adapter
• update the corresponding parse node, request adapter interfaces

Note: this is also impacted by ongoing work in #1411