[RDF] Add internal GetClusterRanges utility to retrieve cluster entry ranges

[siliataider]

This Pull request:

Adds GetClusterRanges as an internal utility to retrieve entry ranges for each cluster in a TTree or RNTuple based RDataFrame.

It returns a list of cluster boundaries across files, using a global offset.

This utility is required by the RDataLoader to shuffle and prefetch data for ML training.

Changes

• RInterfaceBase: make GetLoopManager() public to allow access from internal utilities
• RNTupleDS: add GetClusterRanges as a friend function to access private members fNTupleName and fFileNames
• RNTupleDS: set fNTupleName in the single file constructor (like the multi file constructor)
• RDFUtils: add GetClusterRanges() implementation for both TTree and RNTuple datasources

[siliataider]

@vepadulano the multithreaded approach may not be so straightforward: the global offset depends on the cumulative entry count from previous files, opened sequentially

We could maybe think of a two step approach, collecting clusters in parallel and then sequentially adjusting with a global offset somehow preserving the original order of the files..

[github-actions]

Test Results

    22 files      22 suites   3d 6h 4m 35s ⏱️
 3 831 tests  3 829 ✅  1 💤 1 ❌
76 513 runs  76 494 ✅ 18 💤 1 ❌

For more details on these failures, see this check.

Results for commit 395d045.

♻️ This comment has been updated with latest results.

[pcanal]

The documentation should clearly not that this is a slow operation as it (serially) open all the files and load all the TTree objects (and delete/close them). Those operations can be noticeably slow when done on many (large) remote files. So this function should be used parsimoniously.