Introduce maven dependency management section for plc4x project.

[splatch]

An experimental move of versions to a single place. This allows importing of plc4x parent as BOM for downstream projects.

[chrisdutz]

But as all is the same version anyway.... Not quite sure what the benefit is.

I understand in spring and spring-boot this can be challenging, as everything is in different versions. I particularly like that you instantly know what dependency is an inner-project dependency (the ones with an explicit version) and which ones are not (the ones without).... And for the parent relationship you can't get rid of the versions anyway.