Subscription queries extensions

[sandjelkovic]

To use Subscription queries, gateway methods require ResponseType objects to be used to indicate return types.

To make it Kotlin idiomatic, extension methods on QueryGateway should be implemented to use reified generics, provide a method per ResponseType (Single without a suffix/Optional/Multiple Many) and forward those calls to existing methods on the QueryGateway. The same ResponseType can be used for both Initial and Update response types.

Examples of these types of extensions created for the query method can be found here

This issue is similar to #11 and #16, but for different methods and types of queries.