Allow concatenation of datasets with mixed structs
Fixes #6466 

The idea is to do a recursive check for structs. PyArrow handles it well enough.


For a demo you can do:

```python
from datasets import Dataset, concatenate_datasets

ds = Dataset.from_dict({'speaker': [{'name': 'Ben', 'email': None}]})
ds2 = Dataset.from_dict({'speaker': [{'name': 'Fred', 'email': 'abc@aol.com'}]})
print(concatenate_datasets([ds, ds2]).features)
print(concatenate_datasets([ds, ds2]).to_dict())
```

Now both the features and the rows are fixed.

I note that Sequence suffers from the same problem, so I can fix that in a future PR once this one is merged.