Remember those mysterious Mat type parameters on Source[+Out, +Mat], Flow[-In, +Out, +Mat]
and Sink[-In, +Mat]? They represent the type of values these processing parts return when materialized.
When you chain these together, you can explicitly combine their materialized values. In our example we used
the Keep.right predefined function, which tells the implementation to only care about the materialized type of
the stage currently appended to the right. The materialized type of sumSink is Future[Int] and because of
using Keep.right, the resulting RunnableGraph has also a type parameter of Future[Int].