[ntuple] Manually read RNTuple fields in Streamer()

[silverweed]

...instead of relying on ReadClassBuffer(). This is necessary for schema evolution with RNTuple's custom streamer.

Changes or fixes:

Using TBuffer::ReadClassBuffer in RNTuple::Streamer is wrong because it doesn't handle cases where the schema of RNTuple changes. To do that, we need to manually read back the fields depending on the class version.

Checklist:

• [x] tested changes locally
• [ ] updated the docs (if necessary) - not necessary

[github-actions[bot]]

Test Results

    10 files      10 suites   1d 23h 7m 51s :stopwatch:  2 635 tests  2 635 :white_check_mark: 0 :zzz: 0 :x: 24 868 runs  24 868 :white_check_mark: 0 :zzz: 0 :x:

Results for commit 1de11d83.