[feature request] adding easy to remember `datasets.cache_dataset()` + `datasets.is_dataset_cached()`

Often, there is a need to prepare a dataset but not use it immediately, e.g. think tests suite setup, so it'd be really useful to be able to do:

```
if not datasets.is_dataset_cached(ds): datasets.cache_dataset(ds)
```

This can already be done with:
```
builder = load_dataset_builder(ds)
if not os.path.idsir(builder.cache_dir):
builder.download_and_prepare()
```

but the current way is a way less intuitive and much harder to remember than the proposed API, IMHO.

One more way is to do:

```
_ = load_dataset(ds)
```
but it wastes resources loading the dataset when it's not needed.

this has been discussed at https://huggingface.slack.com/archives/C01229B19EX/p1630021912025800

Thank you!

@lhoestq