fix: migrate to new RDataSource column reader API

[vepadulano]

Change the implementation of the generated RDataSource class to use the newer signature that returns one column reader. This, together with the changes at root-project/root#22234, allow to restore the behaviour that awkward relied upon to store arrays to disk via RDataFrame. Irrespective of these changes, the current in-memory layout of the awkward RecordArray is not fit for storage, leading to writing silently garbage data to disk.

Fixes #3885

[ianna]

@vepadulano - Thank you so much for providing the fix! As we have discussed at the ROOT hackathon this should fix the Snapshot issue and migrate our implementation to the latest ROOT version. So we are (almost) at the state where we were before. I will schedule the PR for merge for the upcoming release. Thanks!

[codecov]

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[ianna]

@ikrommyd - I’m currently actively coding and pushing changes to this branch to help move it forward. Could you hold off on merging main or making updates to it for now? It’s creating a bit of a moving target for my local setup. I'll make sure it's fully synced up once I'm done with this push.