[ntuple] add fMaxKeySize field to RNTuple anchor

[silverweed]

This Pull request:

• Adds a std::uint64_t fMaxKeySize field to the RNTuple struct
• Adds the necessary machinery to serialize that field to the StreamerInfo
• Bumps the RNTuple class version from 4 to 5

TODO

• Update documentation
• Actually use the new field to do header chaining when size exceeds maxKeySize
• Consider adding a fwd compatibility test (read version 5 on disk from version 4 in memory)

Checklist:

• [x] tested changes locally (one test currently fails due to the tested file missing proper StreamerInfo)
• [ ] updated the docs (if necessary)

[github-actions[bot]]

Test Results

    10 files      10 suites   1d 21h 44m 16s :stopwatch:  2 635 tests  2 634 :white_check_mark: 0 :zzz: 1 :x: 24 868 runs  24 860 :white_check_mark: 0 :zzz: 8 :x:

For more details on these failures, see this check.

Results for commit 3408a44d.

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

[silverweed]

Will open a new PR after #15553 is merged