Representing dictionaries (json) objects as features
In the process of adding a new dataset to the hub, I stumbled upon the inability to represent dictionaries that contain different key names, unknown in advance (and may differ between samples), original asked in the [forum](https://discuss.huggingface.co/t/representing-nested-dictionary-with-different-keys/16442).

For instance:

```
sample1 = {"nps": {
  "a": {"id": 0, "text": "text1"},
  "b": {"id": 1, "text": "text2"},
}}
sample2 = {"nps": {
  "a": {"id": 0, "text": "text1"},
  "b": {"id": 1, "text": "text2"},
  "c": {"id": 2, "text": "text3"},
}}
sample3 = {"nps": {
  "a": {"id": 0, "text": "text1"},
  "b": {"id": 1, "text": "text2"},
  "c": {"id": 2, "text": "text3"},
  "d": {"id": 3, "text": "text4"},
}}
```

the `nps` field cannot be represented as a Feature while maintaining its original structure.
@lhoestq suggested to add JSON as a new feature type, which will solve this problem.


It seems like an alternative solution would be to change the original data format, which isn't an optimal solution in my case. Moreover, JSON is a common structure, that will likely to be useful in future datasets as well.