!str #15121 MaterializerSettings now uses a config object

* it's now easier to create a FlowMaterializer(), which will use the default (see akka-streams/.../reference.conf) configuration
* kept methods to easily overload some values of the settings (used to be done via copy())
* made some apply() duplication trade-offs in FlowMaterializer companion for nicer API
* made it possible to set namePrefix from Java API

Resolves #15121
Right, I agree. Implicit ActorSystem would seem good, no? Since this materializer, and it's settings requires an actor system anyway "somewhere". Small change, maybe (like the FlowMaterializer), it should take an `implicit ActorRefFactory` (and blow up if it's not actor context or system). Sounds good?