Build and publish JDK specific artifacts

There are a couple ways to support publishing specific artifacts for multiple JDK versions,

• Multi-release jars (MRJARs): https://openjdk.java.net/jeps/238
• Maven POM classifier: https://maven.apache.org/pom.html#dependencies

This is something that requires investigation and discussion because it isn't totally clear, to me at least, whether the benefits outweigh the costs in additional complexity (both for users and Beam committers).