Add additional ITs for other platforms and integrations

[ascopes]

I'd like to continue to expand the IT pack to include, where possible, examples of using codegen plugins with other languages such as Scala, Clojure, JRuby, Kotlin, Groovy etc.

This issue is to research into what else is available, and then produce ITs that use these plugins.

An example of this: https://github.com/protojure/protoc-plugin