[codegen] Helper function for discriminator mapping allowed values

Created by: sebastien-rosset

Add a helper function that returns a list of allowed discriminator mapping values for a schema object.

The list of all possible schema discriminator mapping values is obtained from explicit discriminator mapping values in the OpenAPI document, and from inherited discriminators through oneOf, allOf, anyOf.

For example, a discriminator may be defined in a 'Pet' schema as shown below. The Dog and Cat schemas inherit the discriminator through the allOf reference. In the 'Pet' schema, the supported discriminator mapping values for the 'objectType' properties are 'Dog' and 'Cat'. * The allowed discriminator mapping value for the Dog schema is 'Dog'. * The allowed discriminator mapping value for the Cat schema is 'Dog'.

Currently, I don't see any helper function that would return a list of allowed discriminator mapping values, such as "Dog" for the Dog schema.

Pet:
   type: object
   discriminator:
     propertyName: objectType
   required:
     - objectType
   properties:
     objectType:
     type: string
 Dog:
   allOf:
   - $ref: '#/components/schemas/Pet'
   - type: object
     properties:
       p1:
        type: string
 Cat:
   allOf:
   - $ref: '#/components/schemas/Pet'
   - type: object
     properties:
       p2:
         type: string

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