[ntuple] Add some internal utilities to the storage layer

[silverweed]

Add a bunch of functions to poke through the abstraction of the storage classes (specifically, to get the underlying TDirectory and RMiniFileReader). These functionalities (all Internal) will be needed by the RNTuple Attributes (see the full PR for details)

Checklist:

• tested changes locally
• updated the docs (if necessary)

[github-actions]

Test Results

    18 files      18 suites   3d 1h 53m 51s ⏱️
 3 668 tests  3 403 ✅   0 💤 265 ❌
64 527 runs  63 925 ✅ 335 💤 267 ❌

For more details on these failures, see this check.

Results for commit 20cba70.

[jblomer]

Do we really need GetUnderlyingDirectory()? Since the attribute anchor is a hidden key, we can just store it in the ROOT file's root directory.

[silverweed]

Do we really need GetUnderlyingDirectory()? Since the attribute anchor is a hidden key, we can just store it in the ROOT file's root directory.

Not sure...the reason I need it in the full PR is to create the AttrSetWriter which then uses it create a PageSinkFile using it. Even if we wanted to store the key in the root directory we'd still need the underlying TFile for it...