Use huggingface_hub.HfApi to list datasets/metrics
Delete `datasets.inspect.HfApi` and use `huggingface_hub.HfApi` instead.

WIP until https://github.com/huggingface/huggingface_hub/pull/429 is merged, then wait for the new release of  `huggingface_hub`, update the `huggingface_hub` version in `setup.py` and merge this PR.

cc: @lhoestq 