Create a sink which materializes into a
CompletionStage which will be completed with a result of the Java 8
Collector transformation and reduction operations.
def javaCollector[T, R](collectorFactory: () => java.util.stream.Collector[T, _ <: Any, R]): Sink[T, Future[R]]
TODO: We would welcome help on contributing descriptions and examples, see: https://github.com/akka/akka/issues/25646