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`

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_5196). All of your documentation changes will be reflected on that endpoint.

@lhoestq I think we should first agree if `datasets` can introduce the breaking change of ignoring `config.HUB_DATASETS_URL`: some users may have override this.

If so, I then would suggest to initiate a deprecation cycle.

After a discussion with the rest of the datasets team, we agreed we can introduce the breaking change of ignoring `config.HUB_DATASETS_URL`: this will have minimal impact, only for **private Hubs**. We will address eventual possible impacts in the future.

Additionally, we also ignore `config.HUB_DEFAULT_VERSION`.

See explanation in this PR description: https://github.com/huggingface/datasets/pull/5196#issue-1434401646

I'm trying to upgrade datasets to 2.7.0 in https://github.com/huggingface/datasets-server, and the tests fail due to this change. I think it's a breaking change (that was not listed in https://github.com/huggingface/datasets/releases/tag/2.7.0) since code that previously worked (by setting `datasets.config.HUB_DATASETS_URL = CI_HUB_DATASETS_URL` for example) does not work anymore.

I'm not sure what is the correct way to set up the tests; besides setting the env var "HF_ENDPOINT" before launching the tests (which, I think, is not a good way to do: the tests should not depend on the environment).

OK, I re-read this thread, and https://github.com/huggingface/datasets/pull/5196#issuecomment-1307430175 explicitely states that `config.HUB_DATASETS_URL` (as well as `config.HUB_DEFAULT_VERSION`) is now ignored. I was expecting the breaking changes to be listed in the release notes: https://github.com/huggingface/datasets/releases/tag/2.7.0.

> I'm not sure what is the correct way to set up the tests; besides setting the env var "HF_ENDPOINT" before launching the tests (which, I think, is not a good way to do: the tests should not depend on the environment).

I think the current workaround of settings an env variable before launching the tests is "not so bad" when considering the fact that env variables are evaluated at import time in `huggingface_hub` (and most probable `datasets` as well). I think that when refactoring this in huggingface_hub (https://github.com/huggingface/huggingface_hub/issues/1172) I'll opt for instantiating a `Settings` object (or `Constants`) that contains all the settings variables. This way it will not be possible to import attributes individually + tests would be easier. As I see it, it would be similar to [what `Pydantic` does](https://pydantic-docs.helpmanual.io/usage/settings/) even though we most probably don't want Pydantic as a root dependency just for that. 

You can use fixtures in your tests:
```python
CI_HUB_ENDPOINT = "https://hub-ci.huggingface.co"
CI_HUB_DATASETS_URL = CI_HUB_ENDPOINT + "/datasets/{repo_id}/resolve/{revision}/{path}"
CI_HFH_HUGGINGFACE_CO_URL_TEMPLATE = CI_HUB_ENDPOINT + "/{repo_id}/resolve/{revision}/{filename}"

@pytest.fixture
def ci_hfh_hf_hub_url(monkeypatch):
    monkeypatch.setattr(
        "huggingface_hub.file_download.HUGGINGFACE_CO_URL_TEMPLATE", CI_HFH_HUGGINGFACE_CO_URL_TEMPLATE
    )

@pytest.fixture
def ci_hub_config(monkeypatch):
    monkeypatch.setattr("datasets.config.HF_ENDPOINT", CI_HUB_ENDPOINT)
    monkeypatch.setattr("datasets.config.HUB_DATASETS_URL", CI_HUB_DATASETS_URL)
```

and use `@pytest.fixture(autouse=True)` if you want to always use the CI endpoints.

And when `huggingface-hub` and `datasets` change the way we can set the endpoint, we'll just need to update the fixtures.
I think ultimately you'll only have to change the `huggingface-hub` endpoint settings


OK.

In fact, in datasets-server we set `config.HUB_DATASETS_URL` (https://github.com/huggingface/datasets-server/blob/35a30dbcd687b26db1f02502ea8305f70c064473/workers/splits/src/splits/config.py#L26) at config time, before starting the workers. It's not an issue with how to launch the tests, but with the app in itself.

I understand that for now, the only way to fix this is to setup `HF_ENDPOINT` in the env when launching the app (currently, we set the endpoint with `COMMON_HF_ENDPOINT`, a custom env var I set to be sure not to have side-effects)

> You can use fixtures in your tests:

Thanks, used in https://github.com/huggingface/datasets-server/pull/644.