Support for official @oneof directive

[NadavK]

@benjie added @oneof to the official GraphQL spec: https://github.com/graphql/graphql-js/releases/tag/v16.9.0
and also to graphl-js: https://github.com/graphql/graphql-js/commits/main/
Can someone kindly port these changes here?

[Cito]

Everything that is added to GraphQL.js will eventually be ported to graphql-core, usually in the order in which it is included in GraphQL.js.

Keeping this open as a reminder to catch up, but normally you don't need to request this as a separate issue.

[fedirz]

Should the issue be closed? It seems like the feature was implemented in b7a18ed

[Cito]

Yes, It's contained in 3.3.0a7 now.