Add key-value extension to DD4hep (fork) and test it for ODD

[paulgessinger]

This would allow us to push this information through to ACTS by attaching information in a generic way.
https://gitlab.cern.ch/acts/OpenDataDetector

A bit of refactoring will be needed for e.g. https://gitlab.cern.ch/acts/OpenDataDetector/-/blob/main/factory/tracker/ODDPixelBarrel_geo.cpp#L250-251, where right now we attached populated material classes, but doesn't have to be the case.

[paulgessinger]

/cc @andresailer

[paulgessinger]

A version of this has been merged to DD4hep, and a WIP PR for using it from ACTS is here: acts-project/acts#1257

[tmadlener]

This will be replaced by the conversion in #36