Offer mechanism to build a distributable with bundled JRE when using Gradle

Offer mechanism to build a distributable with bundled JRE when using Gradle. Ant has this, in the SDK. But not with Gradle. I always kinda figured that Gradle users can figure this one out themselves as one is supposed to anyway tweak around with gradle.build manually. This could be a good feature offered by the SDK.

If there is a good single solution to rule them all, that would be super. The jME initialization wizard currently does includes one solution in its generated script. That would be the easiest solution to just copy that. If there is no single solution, I guess we would need to ask this in the Gradle wizard that what kind of release one is trying to do and figure out which one to use.

One challenge after that is to have the user understand how to trigger this release build. I'm sure it shouldn't happen on every run.

Reference (and options):
https://hub.jmonkeyengine.org/t/how-to-deploy-a-jme-project-as-executable-with-the-integrated-jvm-using-jme-3-6-and-above/47182/

Start by reviewing the jME initialization wizard's generated release script and the linked deployment discussion, then compare that approach with the SDK's existing Ant support and Gradle workflow. Define the supported distributable options and an explicit release-build trigger; done means a Gradle-based project can produce a distributable with a bundled JRE without affecting ordinary run builds.