feat: 🎸 create step dataset-hub-cache
A new step, specific to the Hub (i.e. it will not be backward-compatible), to help the Hub have a cache of the information it needs for each dataset

Note that it's the first step and endpoint that is specific to the Hub. I think we should have more of them (we can use the `-hub` prefix to make it clear in the code). In particular, instead of having N calls from the backend to create the dataset viewer page, we should have only one, well formatted:
- We reduce the number of calls
- We prepare the data beforehand, and it's cached

This PR will be followed by another one that creates the /hub-cache API endpoint. It will replace `/valid`, and will return a paginated list of `dataset-hub-cache` responses, ordered by `updated_at`. We will try to have a SLO of the same order as the other endpoints (85% under 250ms), even if it means having small pages. Anyway, this endpoint will only be used to warm the Hub cache, and then we will switch to the ([still to be implemented](https://github.com/huggingface/datasets-server/issues/1718)) Server-Sent Events endpoint /hub-cache-sse that will send the content of `dataset-hub-cache` on every update.
feat: 🎸 create step dataset-hub-cache: A new step, specific to the Hub (i.e. it will not be backward-compatible), to help the Hub have a cache of the information it needs for each dataset

Note that it's the first step and endpoint that is specific to the Hub. I think we should have more of them (we can use the `-hub` prefix to make it clear in the code). In particular, instead of having N calls from the backend to create the dataset viewer page, we should have only one, well formatted:
- We reduce the number of calls
- We prepare the data beforehand, and it's cached

This PR will be followed by another one that creates the /hub-cache API endpoint. It will replace `/valid`, and will return a paginated list of `dataset-hub-cache` responses, ordered by `updated_at`. We will try to have a SLO of the same order as the other endpoints (85% under 250ms), even if it means having small pages. Anyway, this endpoint will only be used to warm the Hub cache, and then we will switch to the ([still to be implemented](https://github.com/huggingface/datasets-server/issues/1718)) Server-Sent Events endpoint /hub-cache-sse that will send the content of `dataset-hub-cache` on every update.