consider moving `NetexObjectFactory` to `entur/netex-java-model`

[esuomi]

The class in question: no.entur.uttu.export.netex.producer.NetexObjectFactory

That class has several useful utilities for working with the NeTEx model and could work as a good integration interface for the NeTEx Nordic model as a whole.

The reason which prompted this for me is that I'm currently in process of generating organisations.xml in another project and realized this utlity exists, but in a bit unfortunate place dependency wise. I can take inspiration from the most useful parts, of course, to continue on my task, but I think having it paired with the model itself would be beneficial not just for me but everyone else wanting to work with NeTEx Nordic model in general.