First implementation of custom field processor

This is my suggestion for https://github.com/unic/neba/issues/16. It seems to work well. But I'm not very happy about the importance the CustomFieldProcessor has now. It influences the ResourceModelMetaDataRegistrar until the creation of the meta data. Maybe a more elegant solution, where the basic neba components have less touchpoints to the extension can be found. But at least, this is what I expect. With this change, I could simplify a PostProcessor from 250 to 110 LoC. It only contains now the computation of the field value to inject and does not do anything related to reflection or field injection.