[ntuple] Add `Internal::MergeModels`

[enirolf]

The addition of this function is in anticipation for horizontal joining of RNTuples, which will be handled in the context of the RNTupleProcessor that is currently under development (see also #15084).

Checklist:

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

[github-actions[bot]]

Test Results

    11 files      11 suites   1d 21h 54m 38s :stopwatch:  2 629 tests  2 628 :white_check_mark: 0 :zzz: 1 :x: 27 305 runs  27 304 :white_check_mark: 0 :zzz: 1 :x:

For more details on these failures, see this check.

Results for commit 7ed655a0.

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