Enable passing values configuration to GraphQLEnumType as a thunk

[benjie]

Many of the type configuration objects in GraphQL-js have properties that accept thunks:

• GraphQLObjectTypeConfig.fields
• GraphQLObjectTypeConfig.interfaces
• GraphQLInterfaceTypeConfig.fields
• GraphQLInterfaceTypeConfig.interfaces
• GraphQLInputObjectTypeConfig.fields
• GraphQLUnionTypeConfig.types

These are because their definitions may contain references to other types which may form cycles, and thus a thunk is required to enable their definition.

This PR allows setting GraphQLEnumTypeConfig.values to be a thunk. At first, this might seem unnecessary (and, well, it has been this way for 9 years so you can be forgiven for thinking that) but it can be quite useful when building enums that reference types. For example: you might have a field that returns a union (Person.pets), and you might want to add an argument that allows you to limit the returned types to only certain types (Person.pets(only:)):

type Person {
  name: String
  pets(only: [AnimalType!]): [Animal]
}
enum AnimalType {
  Cat
  Dog
  Hamster
  Mouse
}
union AnimalType = Cat | Dog | Hamster | Mouse
type Cat { name: String, owner: Person, numberOfLives: Int }
type Dog implements Animal { name: String, owner: Person, wagsTail: Booean }
type Hamster implements Animal { ... }
type Mouse implements Animal { ... }
type Query {
  currentPerson: Person
}

query {
  currentPerson {
    pets(only: [Cat, Dog]) {
      ... on Cat {
        name
        numberOfLives
      }
      ... on Dog {
        name
        wagsTail
      }
    }
  }
}

For this it can be useful for the AnimalType type to enumerate the types in the Animal union, but those types contain references back to AnimalType -> there's a cycle.

The thunk approach outlined in this PR would solve the cycle. There are workarounds (i.e. manually listing out the types) but they're not as ergonomic as (and are more error-prone than) resolving the values directly from the union.

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