Support multiple discriminator mappings to share one type

[imjn]

Background

Currently, CreateAPI only supports a unique type for each mapping case.
For example, if we have

Container:
      type: object
      required:
        - content
      properties:
        content:
          oneOf:
            - $ref: "#/components/schemas/A"
            - $ref: "#/components/schemas/B"
            - $ref: "#/components/schemas/C"
          discriminator:
            propertyName: kind
            mapping:
              a: "#/components/schemas/A"
              b: "#/components/schemas/B"
              c: "#/components/schemas/C"
              d: "#/components/schemas/A"

it only generates .a but .d is not generated.
This is because we only use the first case with the property type.

CreateAPI/Sources/CreateAPI/Generator/Templates.swift

Line 314 in a59c766

if let correspondingMapping = discriminator.mapping.first(where: { $1 == property.type}) {

How

In this PR, I have fixed the issue above by checking the mapping keys and updated the discriminator test case for the case when two enum cases share one type.

Hope the changes make sense. Let me know if you have better options 🙇🏽

[liamnichols]

Thanks for picking this up! It's looking good 💪💪💪

[liamnichols]

Nice one @imjn, great work! 🚀