Incremental delivery support

[upcFrost]

Hi,

Are there any plans of implementing the incremental delivery (graphql/graphql-spec#742)?
I've spotted that some big frameworks like Apollo started implementing it based on the current draft, so i guess they don't expect it to change that much (https://www.apollographql.com/docs/apollo-server/workflow/requests/#incremental-delivery-experimental).

On the code level, for @stream I guess implementing it would mean turning every resolver into an async generator (in most cases with a single element unless the @stream directive is passed), merging subscription and query field resolution logic together, and leaving the stream pagination logic itself to the resolver.

For @defer, I think it can be implemented by scheduling some of the fields with asyncio.as_completed, gathering and delivering the main output, and then iterating over the rest of the tasks.

I was thinking about implementing these two directives myself, but I'm not 100% sure I'll have enough time in the near future.

Best regards,
Petr