[ntuple] Add some Internal helpers needed for Attributes

[silverweed]

• const version of GetProjectedFieldsOfModel()
• RPageSink::GetUnderlyingDirectory()

These functionalities are needed in the RNTuple Attribute prototype (they are both Internal so there is no long-term implication of adding them).

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    18 files      18 suites   3d 17h 10m 29s ⏱️  2 856 tests  2 856 ✅ 0 💤 0 ❌ 49 966 runs  49 966 ✅ 0 💤 0 ❌

Results for commit 34a5563c.