[ntuple] Support multiple column representations in the merger

[silverweed]

This Pull request:

Significantly reworks the innards of the RNTupleMerger to support fast merging of fields with different but compatible column representations.
Basically it does two things:

• turns all L3 merging cases into L2/L1.
• no longer rejects merging fields with different column representations (previously this was only supported for representations that were the split/unsplit version of each other, and only via L3 merging).

A potentially negative consequence that we might want to revisit is that now the merger won't ever adapt the columns' splitness to the output compression (e.g. if merging changes the source compression from 0 to 505 it will still encode the columns as unsplit, and vice-versa). This will probably be readded in a future PR.

In order to achieve this, some new internal functionality had to be added, most notably RPagePersistentSink::AddColumnRepresentation.

Note that this PR is independent on #21740, which in fact might not be needed at all.

IMPORTANT

This PR introduces our first feature flag and thus the first bump to the specs' major version (1.1.0.0). This means we can now start producing RNTuples which cannot be read by older ROOT versions.

TODO

• check if we need a feature flag for the changes in AddExtendedColumnRanges
• add a test for merging of Real32Trunc/Quant columns with different bit width/value range
• properly split the big merger commit
• update Merging.md

Checklist:

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

[github-actions]

Test Results

    23 files      23 suites   3d 17h 40m 9s ⏱️
 3 872 tests  3 851 ✅ 0 💤 21 ❌
79 695 runs  79 674 ✅ 0 💤 21 ❌

For more details on these failures, see this check.

Results for commit df617cd.

♻️ This comment has been updated with latest results.

[jblomer]

Thank you! That is a huge step forward.

With all the added complexity, I think we need to give the merger another fresh look and try reduce the logic a bit to smaller blocks. For a future PR.

We should remember to backport

• [ntuple] Fix RFieldFundamental::GenerateColumns using the wrong repr idx
• [ntuple] update merger test to make sure we test nRepetitions
• [ntuple] Clarify a bit RFieldBase::EntryToColumnElementIndex
• [ntuple] Introduce feature flag 0 (nested deferred columns)
• Patch up column ids in the header extension (part of [ntuple] Support multiple column representations in the merger)

[silverweed]

We should remember to backport

* [ntuple] Fix RFieldFundamental::GenerateColumns using the wrong repr idx

* [ntuple] update merger test to make sure we test nRepetitions

* [ntuple] Clarify a bit RFieldBase::EntryToColumnElementIndex

* [ntuple] Introduce feature flag 0 (nested deferred columns)

* Patch up column ids in the header extension (part of `[ntuple] Support multiple column representations in the merger`)

Split off those commits into #22655.