Add simple way to disable @defer support

It doesn’t appear like there’s any (simple) way to disable @defer support. @defer support when writing a GraphQL server needs special treatment to send the deferred paths to the client, and in the event that this is not put in place in the code that uses GraphQL-Java, it should be possible to (easily) disable @defer support at the engine level (by ignoring @defer in queries, or failing to validate them).

For now, I can think of only one way to implement this, which is by writing my own instrumentation to either rewrite the Document and/or OperationDefinition to remove @defer in instrumentExecutionContext, or to reject @defer usage altogether in beginValidation (for that case, it might also be possible to make sure the GraphQLSchema does not contain the Directives.DeferDirective; when using SchemaGenerator, this requires duplicating the built schema to remove the directive).

Such instrumentations (or any other way of disabling @defer) should IMO be provided out of the box by GraphQL-Java. WDYT?

[bbakerman]

@defer is not longer on by default in master (which will be released in 13.0 at some stage)

[dinhani]

After finding this example, the following line enabled it again.

schema.transform(builder -> builder.additionalDirective(Directives.DeferDirective));