Add enum_items to discriminated union

[pulasthibandara]

This specifies the discriminator property of the discriminated union as a string literal. As OpenApi doesn't support string literals its common practice to define a literal as an enum of one option. (reference: OAI/OpenAPI-Specification#1313 (comment))

This helps with code generators as it generates a type-safe discriminated union. Following is a Typescript example.

With enum defined:

ex: 

type Animal =
   | { type: 'Dog', bark: string }
   | { type: 'Cat', name: string }

Without enum defined ("type" looses its literals):

ex: 

type Animal =
   | { type: string, bark: string }
   | { type: string, name: string }