Feature Request: GraphQL over SSE

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Description

Implement GraphQL Subscription over SSE (Subscription operation only). Shouldn't be too difficult as to GraphQL over WebSocket. However, I need to support the graphql-sse protocol to be fully client compatible. Not sure whether to support other protocols and client implementation.

Prerequisites

• Enhancement: Integrate Unit Test #2
• Enhancement: API Documentation & Advanced Guide #5
• Feature Request: Subscription Operation #4

Implementation and Plans

• Start with focusing solely on Distinct connections mode on HTTP/2 (require enabling HTTP/2 for akka-http).
• Continue with implementing Single connection mode which require a lot more configuration with the HTTP side of the applications but allow for HTTP/1.1.

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Unit Test Addition

Server-Side Events

1. Able parse query from request.
2. Using a simple schema, able to execute GraphQL Subscription operation using the graphql-sse protocol.
   a. Able to format response as text encoded in next message event.
   b. Able to format response as text encoded in complete message event.
3. Able to handle closing and opening connection.
4. HTTP/2 support for Distinct connection mode.
5. HTTP/1 support for Single connection mode.