add /croissant API endpoint
Added /croissant API

It returns a dataset's JSON Croissant representation.

In particular it contains one recordSet per dataset configuration, with the feature types and parquet files list.

It uses `git+https` instead of pure HTTP locations for the files because Croissant requires hash verifications and they're already provided by git (cc @marcenacp for suggesting this)
Since Croissant implementations might support HTTP more easily than git, we might change to HTTP locations in the future and provide the parquet files hashes.

Datasets that are partially exported to parquet are supported and their recordSet descriptions mention "(first 5GB)".

Unsupported columns are skipped for now, e.g. nested columns (lists, structs), audio etc. (not yet supported in Croissant)
Supported types are strings, integers, and floats.
It is mentioned in the recordSet description which columns are skipped.

Train/test/validation splits are not taken into account yet.

The endpoint will be available at e.g.
```
https://datasets-server.huggingface.co/croissant?dataset=rotten_tomatoes
```

Will add documentation in another PR as soon as the API is stable, and then see how to add it in the dataset pages headers
add /croissant API endpoint: Added /croissant API

It returns a dataset's JSON Croissant representation.

In particular it contains one recordSet per dataset configuration, with the feature types and parquet files list.

It uses `git+https` instead of pure HTTP locations for the files because Croissant requires hash verifications and they're already provided by git (cc @marcenacp for suggesting this)
Since Croissant implementations might support HTTP more easily than git, we might change to HTTP locations in the future and provide the parquet files hashes.

Datasets that are partially exported to parquet are supported and their recordSet descriptions mention "(first 5GB)".

Unsupported columns are skipped for now, e.g. nested columns (lists, structs), audio etc. (not yet supported in Croissant)
Supported types are strings, integers, and floats.
It is mentioned in the recordSet description which columns are skipped.

Train/test/validation splits are not taken into account yet.

The endpoint will be available at e.g.
```
https://datasets-server.huggingface.co/croissant?dataset=rotten_tomatoes
```

Will add documentation in another PR as soon as the API is stable, and then see how to add it in the dataset pages headers