[ntuple] Attributes: update specs and add de/serialization

[silverweed]

First PR of a series to merge the RNTuple Attributes into master. The final result will be this, although the commits will be reorganized to be more coherent and reviewable.

This first PR updates the binary format specification (introducing a new minor version) and updates the Serializer and Descriptor code to match. This is backward-compatible and no Attribute can be written yet since the writer API will be introduced later.

Checklist:

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

[github-actions]

Test Results

    21 files      21 suites   3d 16h 43m 57s ⏱️
 3 664 tests  3 662 ✅  0 💤 2 ❌
75 132 runs  75 119 ✅ 10 💤 3 ❌

For more details on these failures, see this check.

Results for commit 6c3a26f.

♻️ This comment has been updated with latest results.

[jblomer]

In principle looks good to me. Some comments.

[silverweed]

I updated the PR and uniformed the RNTupleAttrSetDescriptor with the other descriptor classes.

[jblomer]

Very nice! Some minor comments.

Since this is changing the binary format, let's perhaps get a second approval.

[hahnjo]

some comments on the spec additions and the iterator implementations