Remove unnecessary classes from oter-extension jar

[worekleszczy]

Currently we have all of the dependencies and scala classes packaged in to otel-extension jar. We should make it as lean as possible - ideally only advice and helper classes - the rest should be provided from otel agent or application code.

[ptrdom]

It is very easy to exclude everything but project code, otel-extension just needs these settings:

assembly / assemblyOption ~= {
  _.withIncludeScala(false)
    .withIncludeDependency(false)
}

Drops .jar size from 53MB to 1MB. But then every required Akka and Scala class needs to be added to helper classes and without muzzle it would be quite frustrating to do. Let's postpone this until we have moved to Gradle and can use muzzle.