Add support for Google App Engine Java11 standard environment

[marctatham]

Added & configured the Google App Engine Gradle plugin

As part of this I've also optimized the build to only build the UberJar using the shadowJar task (instead of building the standard jar with the jar task as well)

Please note:

• The appengine set of tasks of course require some setup. Please complete the steps listed in before you begin to be able to test this
• The Google Cloud plugin for IntelliJ is NOT required for this in any way, at this time is useful for apps running on the java8 standard environment, but not useful for apps running on the java11 standard environment.

Running ./gradlew appengineDeploy results in the following

This is of course accessible then via your PROJET-ID.appspot.com url which is output upon successful completion of the above gradlew command - alternatively you can find it by executing the Google Cloud CLI command gcloud app browse