[ntuple] Make RNTupleFileWriter::Recreate/Append return unique_ptr

[silverweed]

Also apply clang-format to the involved files in the process.

Changes or fixes:

RNTupleFileWriter::Recreate and RNTupleFileWriter::Append now return a unique_ptr rather than a raw pointer, to make it obvious that the caller gets ownership of that pointer.

Checklist:

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

This PR fixes #

[github-actions[bot]]

Test Results

    11 files      11 suites   2d 4h 50m 24s :stopwatch:  2 637 tests  2 636 :white_check_mark: 0 :zzz: 1 :x: 27 442 runs  27 441 :white_check_mark: 0 :zzz: 1 :x:

For more details on these failures, see this check.

Results for commit 97bd27e4.

:recycle: This comment has been updated with latest results.