NTuple service example

We have some examples here that use the histogram service. Do we have similar examples (or can we come up with examples) that let us produce ntuples (outside of EDM4hep)? Would make some monitoring tasks a bit easier potentially. At the moment one would have to create a `podio::UserDataCollection` for every branch that we want to have in the ntuple which again requires changing the algorithm type in some cases.

Start by reviewing the existing histogram service examples and the current use of podio::UserDataCollection for ntuple branches. Determine what an ntuple example outside EDM4hep should demonstrate and how it avoids requiring a UserDataCollection for every branch or changing algorithm types. Done means a documented, working example or a clearly scoped service approach for the monitoring use case.

Written by the indexing model from the issue text.