Maven Archetype for Annotation Processor

[dbluhm]

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We should include a maven archetype for a new Annotation Processor module that utilizes our annotation processing tools.

The produced maven project should include:

• A named AnnotationProcessor class extending AbstractAnnotationProcessor, also annotated with Google's AutoService annotations to simplify integration of the processor into other projects.
• An example AnnotationExtractor extending AbstractAnnotationExtractor and a corresponding example Annotation Metadata class.
• An example GeneratedFileWriter (and link to VelocitySourceWriter example or include one directly?).
• An example test of the processor using Google's compile-testing library.