Preserve tree structure when loading HDF5

### Feature request

https://github.com/huggingface/datasets/pull/7740#discussion_r2285605374

### Motivation

`datasets` has the `Features` class for representing nested features. HDF5 files have groups of datasets which are nested, though in #7690 the keys are flattened. We should preserve that structure for the user.

### Your contribution

I'll open a PR (#7743)