Akka.Streams: make it easier to pass in a friendly LogSource name

[Aaronontheweb]

Is your feature request related to a problem? Please describe.

Right now the built in .Log method in Akka.Streams prints out a lot of junk under the LogSource of one of the built-in materialized actors. It should be easier to pass in a human-friendly log source name as an ActorAttribute when organizing the stream.

Describe the solution you'd like

var task = source.DivertTo(evenSink, i => i % 2 == 0)
.AddAttributes(ActorAttributes.CreateLogSource(...))
.DivertTo(threeSink, i => i % 3 == 0)
.DivertTo(fiveSink, i => i % 5 == 0)
.RunWith(Sink.ForEach<int>(i => Console.WriteLine($"{i} is unhandled")), actorSystem);

Describe alternatives you've considered

I'm going to make a separate issue about the .Log method itself in Akka.Streams - maybe we can make that method customizable instead.