[java] add JDK version option

[jkutner]

Add a mechanism to select the JDK version and distribution.

Heroku uses a system.properties file with contents like:

java.runtime.version=1.8.0_172

For more info see Heroku docs.

/cc @cmoulliard

[jromero]

We're always looking for contributions. :)

A good reference point would be our Ruby tutorial: https://buildpacks.io/docs/create-buildpack/make-buildpack-configurable/