In /valid, associate an enum to every dataset
Instead of a list of datasets:

```json
{
  "valid": [
    "0-hero/OIG-small-chip2",
    "000alen/semantic",
    "04-07-22/wep-probes",
    "0721boy/nva-pic",
    ...
  ]
}
```

we could return an object, where the datasets are the keys, and the values are lists of available features: `"preview" | "parquet" | "parquet-sample"`

```json
{
  "0-hero/OIG-small-chip2": ["preview"],
  "000alen/semantic": ["preview", "parquet"],
  "04-07-22/wep-probes": ["preview", "parquet-sample"],
  "0721boy/nva-pic": [], 
  ...
}
```

The list of features for each dataset helps the Hub show the appropriate text/icons/UI.

Note that we might want to change the name of the endpoint to avoid breaking change, and to be more descriptive: `/features-by-dataset` (I don't like it, because `features` has another meaning in datasets).

Also: the response could take too long to be computed online as it's currently done (it already takes about 300ms to compute), so we should precompute it regularly with a cronjob and cache the result. See #891 
In /valid, associate an enum to every dataset: Instead of a list of datasets:

```json
{
  "valid": [
    "0-hero/OIG-small-chip2",
    "000alen/semantic",
    "04-07-22/wep-probes",
    "0721boy/nva-pic",
    ...
  ]
}
```

we could return an object, where the datasets are the keys, and the values are lists of available features: `"preview" | "parquet" | "parquet-sample"`

```json
{
  "0-hero/OIG-small-chip2": ["preview"],
  "000alen/semantic": ["preview", "parquet"],
  "04-07-22/wep-probes": ["preview", "parquet-sample"],
  "0721boy/nva-pic": [], 
  ...
}
```

The list of features for each dataset helps the Hub show the appropriate text/icons/UI.

Note that we might want to change the name of the endpoint to avoid breaking change, and to be more descriptive: `/features-by-dataset` (I don't like it, because `features` has another meaning in datasets).

Also: the response could take too long to be computed online as it's currently done (it already takes about 300ms to compute), so we should precompute it regularly with a cronjob and cache the result. See #891 