Support GraphQL Subscriptions

[constantind]

Add GraphQL Subscriptions support:

https://github.com/graphql/graphql-spec/blob/June2018/rfcs/Subscriptions.md

Environment Details

• Helidon Version: supported
• Helidon SE or Helidon MP: both
• JDK version: supported
• OS: any
• Docker version (if applicable):

---

Problem Description

Currently graphql subscription operations are not supported: microprofile/microprofile-graphql#174

Servers that support:
DGS, Apollo Kotlin and Spring GraphQL support GraphQL subscriptions over WebSockets and SSE
Protocol Specs:
SSE: https://github.com/enisdenjo/graphql-sse/blob/master/PROTOCOL.md
Websocket: https://github.com/enisdenjo/graphql-ws/blob/master/PROTOCOL.md

Reactive clients:
Apollo client, DGS client, Spring Graphql client supported