RDF ConverterPlugin for metadatagroups

[floriangantner]

Is your feature request related to a problem? Please describe.
Currently the RDF-Generating MetadataConverterPlugin in DSpace7 can only generate text-based values from metadatavalues.
https://github.com/4Science/DSpace/blob/bf5b19b6da561a9fc2e088dc1b376987c8115760/dspace-api/src/main/java/org/dspace/rdf/conversion/MetadataConverterPlugin.java#L159C6-L159C6

DSpace-Cris often hold metadatagroups containing metadata belongin together. This cannot be expressed in the current ConverterPlugin.
Because the normale Dspace does not use metadatagroup we see this as some DSpace-Cris problem.

Describe the solution you'd like
We would like some ConverterPlugin which considers metadatagroups. It should generate some blank node which groups all metadata belongin together and considers missing placeholders.

Describe alternatives or workarounds you've considered
It could also be possible to use some xml-Crosswalk and corresponding generators to generate the rdf.
This problem mainly concerns you if you want to reuse your data as rdf in other systems or fill some triple store with relevant data.

Additional context
Add any other context or screenshots about the feature request here.