[Maven] Wrap the generator as a maven plugin so that users don't have to install and build the project in order to use it

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Start by inspecting the Java generator and its JSON-configuration entry point, then determine how the project is currently built and invoked. The issue provides no files, tests, or acceptance criteria, so clarify the Maven plugin's scope before implementation; done should mean users can invoke the generator through Maven without separately installing and building the project.

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