Support subscriptions

GraphQL Java can support subscriptions: https://www.graphql-java.com/documentation/v11/subscriptions/

We just need to come up with a good way to represent the different type in the schema.

This may be able to be done without a breaking change. Can we just add a @GraphQLSubscription?

[smyrick]

@shahabio I had started working on the update but had trouble getting the example app working with spring websockets. If you want to pick up from my branch you can pull from here: https://github.com/smyrick/graphql-kotlin/tree/subscribtions

[hinterdupfinger]

putting

override fun getSubProtocols(): List<String> {
    return listOf("graphql-ws")
}

in SubscriptionHandler(private val graphQL: GraphQL) : WebSocketHandler

should fix the the protocol issue as org.springframework.web.reactive.socket.server.support.HandshakeWebSocketService is checking for supported protocols in the handler