Add Scala cross-compilation

Now that the scala-concurrent-plugin is merged, it may be a good idea to investigate cross-compilation first before adding new libraries.

I have no experience with Maven whatsoever so it is unclear to me what is possible and what not.
The Maven Scala plugin describes how to do cross-compilation in a Maven project as follows:
https://github.com/davidB/scala-maven-plugin/wiki/Frequently-Asked-Questions#cross-building-with-maven

To give you an idea on how tricky handling different versions of Scala is, here is an example of the datadog-java-agent handling it with Gradle: https://github.com/DataDog/dd-trace-java/blob/master/dd-java-agent/instrumentation/akka-http-10.0/akka-http-10.0.gradle

Some features the ElasticAPM should be able to do:
* Test against different versions of Scala
* Test against different versions of a dependency