Add support for Groovy

[Nava2]

Currently we only support Java as a generation language. However, due to the assertj-generator operating on TypeToken<> rather than actual source files, we can generate java code from any compiled jvm sources.

To do this, we need to adjust the Plugin's registration to look at other, non-Java plugins (i.e. Kotlin plugin or Groovy). It will still require the Java plugin to function, but most JVM languages do today as they use the SourceSet mechanisms.

Duplicated from #48 to cover groovy.