fix: Selection set initializers for fragment type cases

[calvincestari]

Related to #3482.

When a fragment type case is declared on a union that does not matches the field type the selection set initializer parameters are missing. See sample below and take note of the size field types and the type case used on the BigOrSmallFragment declaration. In the generated Swift code you will also notice that the Animal.AsCat.Size.AsBigOrSmall initializer properties are missing.

There are two possible fixes (2 is preferred):

1. Generate the missing properties into the initializer
2. Generate a fragment initializer instead of the selection fields

Schema:

type Query {
  zoo: Zoo
}

type Zoo {
  description: String!
  animals: [Animal]
}

type Dog {
  size: BigOrSmall!
}

type Cat {
  size: BaseSize!
}

type Big {
  height: Int!
}

type Small {
  height: Int!
}

union Animal = Dog | Cat

union BigOrSmall = Big | Small

union BaseSize = Big | Small

Operation:

query Zoo {
  zoo {
    ...ZooFragment
  }
}

fragment ZooFragment on Zoo {
  description
  animals {
    ... on Dog {
      size {
        ...BigOrSmallFragment
      }
    }
    ... on Cat {
      size {
        ...BigOrSmallFragment
      }
    }
  }
}

fragment BigOrSmallFragment on BigOrSmall {
  ...BigFragment
  ...SmallFragment
}

fragment BigFragment on Big {
  height
}

fragment SmallFragment on Small {
  height
}