feat: add RNTuple with multiple cluster groups

[ariostas]

This PR adds a file to test scikit-hep/uproot5#1357. I'm still working on cleaning up some of the reading code, so I might end up adding more files to this PR.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 72.38%. Comparing base (797693e) to head (815e95b).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #168   +/-   ##
=======================================
  Coverage   72.38%   72.38%           
=======================================
  Files           3        3           
  Lines         134      134           
=======================================
  Hits           97       97           
  Misses         37       37           

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