MegaIssue: GraphQL spec tracking

Keep on with the bleeding edge of the GraphQL RFCs:

* [x] service capabilities
* [x] `onError`
* [x] `@defer`
* [x] `@stream`
* [x] Fields extensions
* [x] `@semanticNonNull`, `@catch`
* [ ] fragment arguments
* [ ] HTTP
* [ ] `@requiresOptIn`
* ...