!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
Please add `\n` at the end of the file. This [could lead](https://github.com/akka/akka/pull/15718/files#r16704974) to test failures in multi-node tests.