[gateway] Websocket subprotocol isn't forwarded to the endpoint

[gaetanmaisse]

💥 Describe the bug

Websocket subprotocol isn't forwarded to the endpoint causing a 502 error when trying to connect.

🌄 To Reproduce

Steps to reproduce the behaviour:

1. Clone https://github.com/apollographql/docs-examples/tree/main/apollo-server/v3/subscriptions-graphql-ws
2. Go inside the directory and run yarn && yarn start, it will start the WS server on: ws://localhost:4000/graphql
3. Using Postman you should be able to connect to ws://localhost:4000/graphql, set the header Sec-WebSocket-Protocol: graphql-transport-ws and send the following message (within 3s otherwise you will be disconnected):

{ 
  "type": "connection_init" 
}

You will receive:

{ 
  "type": "connection_ack" 
}

4. Activate Websocket in the gravitee.yml of the gateway:

http:
  websocket:
    enabled: true
    subProtocols: graphql-transport-ws
    perMessageWebSocketCompressionSupported: true
    perFrameWebSocketCompressionSupported: true

4. On Gravitee APIM, create an API with ws://localhost:4000/graphql as a backend endpoint
5. Try to connect to it using the same method as in 3.
6. 💥 You will receive an error: Error: Unexpected server response: 502

🌈 Expected behaviour

Unable to connect, graphql-transport-ws protocol isn't forward

Current behaviour

The connection should be ok

🔗 Dependencies

Also related to #8228

[gaetanmaisse]

Tested on 3.15.x, it looks ok, subprotocol is now handled. But it appears that connecting to WS server behind a redirection isn't easy... it's working with Postman but wasn't able to connect using curl, ws, wscat 🤷🏻