Google FCM: become Alpakka 1.0 ready

[ennru]

• Move internal code to impl package
• Mark internal code @InternalApi and private[connector]
• Move documentation example code to packages docs.javadsl and docs.scaladsl
• Make settings to be non-case classes

[tg44]

Hey!

Moving documentation code means; from the tests to the docs?

Why we make settings to non-case classes?

BTW this component has more todo-s. Most of the fields must be option (which was not described in the fcm-docs but came out in real use), the akka-docs should describe some retry mechanism, and use-cases.

[ennru]

The example code should be moved to another package. Still to be run as tests. That way we can make sure the docs don't reference any internal classes.

Case-classes cannot be evolved in a binary-compatible way.

Please add issues for other things that should be fixed before we promote the module to being API-stable.