Configs and splits
See https://github.com/huggingface/datasets-server/issues/701

This PR does:
- create a new endpoint `/config-names`. It gives the list of config names for a dataset
- create a new endpoint `/split-names`. It gives the list of split names for a config (not for a dataset, which is the difference with /splits for now). Note that /split-names, as /splits, may depend on the streaming mode and might fail for this reason.
- introduce a new "input-type": "config" in the processing steps. Before, a processing step was "dataset" (the input is a  dataset name) or "split" (the inputs are dataset, config, and split). Now, we enable "config" (the inputs are dataset and config) for the new endpoint `/split-names`.

Once this PR is merged, the plan is to:
- fill the cache for the two new endpoints for all the datasets on the Hub (might take some hours to complete),
- create a PR on moonlanding to use these two new endpoints instead of `/splits`
- remove the `/splits` processing step, delete the cache for this endpoint, make the endpoint an alias to `/split-names` and add the ability for `/split-names` to concatenate the responses for all the configs if the only parameter is `dataset` (no `config` passed)
Configs and splits: See https://github.com/huggingface/datasets-server/issues/701

This PR does:
- create a new endpoint `/config-names`. It gives the list of config names for a dataset
- create a new endpoint `/split-names`. It gives the list of split names for a config (not for a dataset, which is the difference with /splits for now). Note that /split-names, as /splits, may depend on the streaming mode and might fail for this reason.
- introduce a new "input-type": "config" in the processing steps. Before, a processing step was "dataset" (the input is a  dataset name) or "split" (the inputs are dataset, config, and split). Now, we enable "config" (the inputs are dataset and config) for the new endpoint `/split-names`.

Once this PR is merged, the plan is to:
- fill the cache for the two new endpoints for all the datasets on the Hub (might take some hours to complete),
- create a PR on moonlanding to use these two new endpoints instead of `/splits`
- remove the `/splits` processing step, delete the cache for this endpoint, make the endpoint an alias to `/split-names` and add the ability for `/split-names` to concatenate the responses for all the configs if the only parameter is `dataset` (no `config` passed)