[ntuple] Allow keys larger than 1GB in RNTuple

[silverweed]

This Pull request:

uses RNTuple::fMaxKeySize to split any payload larger than that into multiple chunks, each backed by a TKey. These keys are then read back as a single object.

Depends on #15869

Checklist:

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

[github-actions[bot]]

Test Results

    13 files      13 suites   2d 20h 38m 29s :stopwatch:  2 651 tests  2 651 :white_check_mark: 0 :zzz: 0 :x: 32 645 runs  32 645 :white_check_mark: 0 :zzz: 0 :x:

Results for commit c9429ffa.

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