JUnit5 option for FDK users. #140

We set our JUnit version to 4.12 here https://github.com/fnproject/fdk-java/blob/master/pom.xml#L31

Typically doing that is fine because junit is normally scoped to testing, and users can be free to choose their version as ours won't be included transitively into their builds. However we have a testing module which has junit as a normally-scoped dep: https://github.com/fnproject/fdk-java/blob/master/testing/pom.xml#L25-L29

We should support users wanting to use JUnit 5, and we should also track JUnit releases.