Support discriminator for polymorphic types and support oneOf

[bowenwr]

Is your feature request related to a problem? Please describe.

When an endpoint returns oneOf several different classes, currently it returns an empty object. Sample component snippet:

    ReturnedObject:
      oneOf:
        - $ref: '#/components/schemas/PolymorphicType'
        - $ref: '#/components/schemas/AnotherPolymorphicType'

Describe the solution you'd like

Support discriminators to be able to deserialize polymorphic types.

[dbanty]

I tested this with the latest code on main (roughly 0.7.0-rc.1) and it does attempt to generate proper responses now using the same code it would for unions inside a model (basically a series of try/except). So it's better than nothing, though for sure we still should add discriminators in a future version.