Use hfh hf_hub_url function
Small refactoring to use `hf_hub_url` function from `huggingface_hub`.

This PR also creates the `hub` module that will contain all `huggingface_hub` functionalities relevant to `datasets`.

This is a necessary stage before implementing the use of the `hfh` caching system (which uses its `hf_hub_url` under the hood).

EDIT:
~~Finally, we use our `config.HUB_DATASETS_URL` when using `hfh.hf_hub_url`~~
There is a breaking change:  the `hfh` `hf_hub_url` function uses
- `hfh` `HUGGINGFACE_CO_URL_TEMPLATE` URL template, different from the `datasets` `config.HUB_DATASETS_URL`
- also, `hfh` `DEFAULT_REVISION`, instead of `datasets` `config.HUB_DEFAULT_VERSION`