[ntuple] Add RPageSinkFile::CloneWithDifferentName

[silverweed]

Last part of #19904.

Adds a writing analogue to RPageSourceFile::OpenWithDifferentAnchor, which allows to derive a new Sink from an existing one using the same underlying directory but writing to a different RNTuple.
Was previously called DeriveFor but I found the name too obscure and I think this is clearer.

Checklist:

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

[github-actions]

Test Results

    22 files      22 suites   3d 9h 22m 25s ⏱️
 3 776 tests  3 774 ✅ 0 💤 2 ❌
75 119 runs  75 117 ✅ 0 💤 2 ❌

For more details on these failures, see this check.

Results for commit c8e2b81.

♻️ This comment has been updated with latest results.

[pcanal]

which allows to derive a new Sink from an existing one using the same underlying directory but writing to a different RNTuple.

Do we also need to support the case where the user would like to clone in a different directory (and/or is it mostly for internal use)?

[silverweed]

which allows to derive a new Sink from an existing one using the same underlying directory but writing to a different RNTuple.

Do we also need to support the case where the user would like to clone in a different directory (and/or is it mostly for internal use)?

It's for internal use only (for the Attributes) and we always want to write to the same directory