Created an example app

[LukaszKontowski]

• example-app runs properly with cluster nodes started from separate terminal windows
• example-app runs (or just compiles?) as a part of standard ci. If it should run - then logic should be changed so that it runs for a short time and then a shutdown is performed
• README and GUIDE updated (tell users about this example app)
• logic for dump-persistence-schema added (currently missing) - will be done within Add real dumping of persistence-schema to examples/ #216
• .scala-steward.conf updated with example-app
• this particular example project is built in the CI workflow
• both example projects are scalafmt-checked just as the top-level project (in CI)
• MAYBE: change example-app logic? (now it is mostly copied from akka-samples-cluster-scala ...) Not needed, we can use it.
• MAYBE: CI updated so that after release is published - ASH version in examples/example-app/build.sbt is bumped automatically? (or maybe Scala Steward will do it afterwards in it's PR?) (scala steward should do it - we'll see)

[LukaszKontowski]

to be done:

• change example-app logic maybe? (now it just mostly copied from akka-samples-cluster-scala ...)
• make example-app run (or just compile?) as a part of standard ci - if run - then logic should be changed so that it runs for a short time and then a shutdown is performed
• update README and GUIDE - tell users about this example app
• add logic for dump-persistence-schema (currently missing)
• update CI so that after release is published - ASH version in examples/example-app/build.sbt is bumped automatically (or maybe Scala Steward will do it afterwards in it's PR?)
• • all Pawel's suggestions

[PawelLipski]

to be done:

Btw you can move it to PR description, and use the checkboxes:

- [ ] change example-app logic maybe? (now it just mostly copied from akka-samples-cluster-scala ...)
- [ ] ....

This way you can tick off the stuff that's done

[LukaszKontowski]

Code from the https://github.com/akka/akka-samples/tree/2.6/akka-sample-cluster-scala is in following files:

examples/example-app/src/main/scala/org/virtuslab/example/App.scala
examples/example-app/src/main/scala/org/virtuslab/example/StatsClient.scala
examples/example-app/src/main/scala/org/virtuslab/example/StatsService.scala
examples/example-app/src/main/scala/org/virtuslab/example/StatsWorker.scala
examples/example-app/src/main/resources/logback.xml
examples/example-app/src/main/resources/example.conf

( // our code comments added for our pieces of code )

All the rest is ours.