Add support for Apollo-style batched query requests

[marceloverdijk]

See:

• https://www.apollographql.com/docs/apollo-server/requests/#postRequests
• https://www.prisma.io/blog/improving-performance-with-apollo-query-batching-66455ea9d8b

[marceloverdijk]

Putting this on hold to consider whether to add non-official GraphQL spec related functionality or not.

[benmccann]

Here's a related ticket to follow: https://github.com/graphql/graphql-spec/issues/517

[donbeave]

@marceloverdijk @ilopmar Maybe can introduce it as a separate experimental controller? If yes, I can add PR with changes that will add support: https://www.apollographql.com/docs/link/links/batch-http/.