Add maven wrapper

[DaGeRe]

With a maven wrapper, its much easier to have reproducible builds on different machines, so I'd recommend to add one to the project.

[SimonEismann]

Looks cool, could you add some documentation for how to use this?

[DaGeRe]

Sure, but its very easy: Just use ./mvnw instead of mvn to run maven from the command line (so maven doesn't need to be installed any longer, and no dependence on the maven system version is present). I changed it in GET_STARTED and also in the workflows.