multipart/mixed transport support for deferred queries

[giulio-opal]

@defer spec doesn't necessarily specify which request headers need to be supported, but Apollo Client injects a multipart/mixed header whenever the query contains a @defer directive and expects a multipart/mixed response:

• https://github.com/apollographql/apollo-client/blob/main/src/link/http/createHttpLink.ts#L193-L194

Spec: https://github.com/graphql/graphql-wg/blob/e4ef5f9d5997815d9de6681655c152b6b7838b4c/rfcs/DeferStream.md
This partially solves: #3335 and #2771

I have:

• Added tests covering the bug / feature (see testing)
• Updated any relevant documentation (see docs)

Note that the response is actually wrapped in a incremental property because that's how the Apollo Client expects it: https://github.com/apollographql/apollo-client/blob/v3.11.8/src/link/http/__tests__/responseIterator.ts#L79-L91

Example of communication (redacted few fields):

$ curl 'http://localhost:3000/query' -H 'Authorization: bearer xxx==' -H 'Connection: keep-alive' -H 'accept: multipart/mixed' -H 'content-type: application/json' --data-raw '{"operationName":"InventoryPage","variables":{},"query":"query InventoryPage {\n  apps {\n    apps {\n      id\n      ...AppGroupCountCell @defer(label: \"AppGroupCountCell\")\n      ...AppUserCountCell @defer(label: \"AppUserCountCell\")\n      __typename\n    }\n    __typename\n  }\n}\n\nfragment AppGroupCountCell on App {\n  groupAccessCount\n  __typename\n}\n\nfragment AppUserCountCell on App {\n  userAccessCount\n  __typename\n}\n"}'
--graphql
Content-Type: application/json

{"data":{"apps":{"apps":[{"id":"123","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"123","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"123","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"123","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"123","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"1324","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"sdf","groupAccessCount":null,"__typename":"App","userAccessCount":null},{"id":"asdf","groupAccessCount":null,"__typename":"App","userAccessCount":null}],"__typename":"AppsOutput"}},"hasNext":true}

--graphql
Content-Type: application/json

{"incremental":[{"data":{"groupAccessCount":0},"label":"AppGroupCountCell","path":["apps","apps",6],"hasNext":true}],"hasNext":true}

--graphql
Content-Type: application/json

{"incremental":[{"data":{"groupAccessCount":0},"label":"AppGroupCountCell","path":["apps","apps",23],"hasNext":true}],"hasNext":true}

--graphql
Content-Type: application/json

{"incremental":[{"data":{"groupAccessCount":0},"label":"AppGroupCountCell","path":["apps","apps",4],"hasNext":true}],"hasNext":true}

...

[coveralls]

coverage: 59.267% (+0.4%) from 58.893%
when pulling ec8dffd on giulio-opal:giulio/defer-over-multipart
into acd9b6a on 99designs:master.