Handle Apollo subscription onConnect

[oliemansm]

Introduced a ConnectionSubscriptionListener that provides the ability to handle the onConnect. See also: https://www.apollographql.com/docs/graphql-subscriptions/authentication.html.

If onConnect returns an Object it is set in the Session.userProperties, which in turn is used now when building the GraphQLContext. It's currently available through GraphQLContext.getConnectResult().

GraphQLContext could be improved by splitting it up in separate context implementations instead of this version, but left that for a different case.

The default implementation provided here does nothing. If you want to add authorization using these features you would have to provide an implementation of ApolloSubscriptionConnectionListener that would handle the authorization and throws a RuntimeException if authorization wasn't successful. If the user could be authenticated it could return a custom object representing the principal which in turn would be made available in the GraphQLContext.

By providing your own custom GraphQLContextBuilder as well you could align it with your own user types obviously.