Consider adding SSE for Subscription operations to the specification

Server-Sent Events (SSE) is a server push technology enabling a client to receive automatic updates from a server via an HTTP connection, that does not require any complex protocol compared to WebSocket, thus making it a perfect fit for having a recommended and standardized way of doing GraphQL Subscriptions in the GraphQL over HTTP specification.

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- https://github.com/enisdenjo/graphql-sse
- https://www.graphql-yoga.com/docs/features/subscriptions