Adds support for custom enum value resolvers

[orta]

We were seeing some issues around enums in our stitched API, artsy/metaphysics#836, and this looked like the most likely candidate.

We built off @clee681's work mainly, wouldn't have known where to start without #364

This might fail because it's waiting on DefinitelyTyped/DefinitelyTyped#21786 - if you'd accept a temporary (thing as any) I can un-block that.

• If this PR is a new feature, reference an issue where a consensus about the design was reached (not necessary for small changes) Add support for custom Enum values in schema #363
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass
• Update CHANGELOG.md with your change

/cc @sweir27 @alloy

[apollo-cla]

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[orta]

Perhaps this any is a bit too open? (Looks like it triggers a bunch of additional errors later)

[stubailo]

Could this just be GraphQLEnumType?

[orta]

I don't think so, that's a full class and really this wants to represent the custom value lookup system which is currently [key: string]: string.

I think I'm gonna go with:

export type IEnumResolver = { [key: string]: string };
export interface IResolvers {
  [key: string]: (() => any) | IResolverObject | GraphQLScalarType | IEnumResolver;
}

Which tightens it, and doesn't create the extra errors in tests

[stubailo]

I'm not really sure how this works, since the instanceof check seems like it would always fail, right?

[stubailo]

Could this just be GraphQLEnumType?

[stubailo]

Wait I'm really confused how this works with resolvers like:

Color: {
  RED: '#EA3232'
},

Since:

{
  RED: '#EA3232'
} instanceof GraphQLEnumType

Is definitely false, right?

[orta]

Yeah, I'd agree - but I don't think that's what's being checked for here ( this code is definitely being ran ) when I tried printing out type here it says "Color" which I assume is the description on that class ( I'm still a little new to JS )

[stubailo]

Can we add the (thing as any) temporarily with a TODO comment, and maybe an issue? Curious if the tests pass otherwise.

[stubailo]

Thank you!

[orta]

<3 thank you