So you need to be able to provide an interface in your main project and then allow users to implement this interface when they want and to be able to feed the implementation to your main project.
1. Build your implementation with maven
2. Take the resulting jar from target and copy it into your impl folder under your main project target folder
3. Start your project with:
java -classpath "demo-0.0.1-SNAPSHOT.jar;imp*" demo.Application