Use discriminator and mapping object for `resolveType` in GraphQL union types

[Discriminator and mapping objects](https://github.com/OAI/OpenAPI-Specification/blob/master/versions/3.1.0.md#discriminator-object) are used to distinguish between different schemas utilized in `oneOf` or `anyOf`. This can be incorporated into the `resolveType` function used to distinguish member types in GraphQL union types.