Support updated rhdf5 handling of nullable vectors and boolean enums (e.g., ordered factors)

[mtmorgan]

The updated code is available at https://github.com/grimbough/rhdf5/tree/scverse-hackathon e.g., BiocManager::install("grimbough/rhdf5", ref = "scverse-hackathon")

nullable vectors are read directly, no need to separately read 'value' and 'mask'

boolean enum attributes are returned as scalar logical