kogito-codegen-tests: alternative testing strategy for codegen

Currently we are doing dynamic compilation and classloading to test code generation. I am proposing an alternative strategy that use the plain Maven plugin.

• create `kogito-codegen-tests`
• write data/rules/process sources under src/main/ {java,resources}
• write tests under src/test/

tests are now allowed to refer directly to codegenerated classes by name.
just use mvn compile test to run the tests

caveat: not sure if the maven plugin is picked up from the current source tree or from the SNAPSHOT on the nexus, we need to check this