feat: create subsciption over multipart

[df-wg]

trafficstars

Motivation and Context

One of the methods that Apollo recommends using is HTTP Multipart for subscriptions, as described here and here

This PR enables the router to follow the same protocol, so that it can respond in the format:

--graphql
content-type: application/json

{}
--graphql
content-type: application/json

{"payload":{"data":{"productPriceChanged":{"name":"Croissant","price":400,"reviews":[{"score":5}]}}}}
--graphql
content-type: application/json

{"payload":{"data":{"productPriceChanged":{"name":"Croissant","price":375,"reviews":[{"score":5}]}}}}
--graphql
content-type: application/json

{"payload":{"data":{"productPriceChanged":{"name":"Croissant","price":425,"reviews":[{"score":5}]}}}}
--graphql--

TODO

• [ ] Tests or benchmark included
• [ ] Documentation is changed or added on https://app.gitbook.com/
• [ ] PR title must follow conventional-commit-standard