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.

Comments:
The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/datasets/pr_6587). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

friendly bump

<details>
<summary>Show benchmarks</summary>

PyArrow==8.0.0

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005403 / 0.011353 (-0.005950) | 0.003807 / 0.011008 (-0.007201) | 0.063850 / 0.038508 (0.025342) | 0.028242 / 0.023109 (0.005132) | 0.242866 / 0.275898 (-0.033032) | 0.266015 / 0.323480 (-0.057464) | 0.004111 / 0.007986 (-0.003875) | 0.002816 / 0.004328 (-0.001513) | 0.048862 / 0.004250 (0.044611) | 0.043036 / 0.037052 (0.005984) | 0.255149 / 0.258489 (-0.003340) | 0.280105 / 0.293841 (-0.013736) | 0.028182 / 0.128546 (-0.100365) | 0.010997 / 0.075646 (-0.064649) | 0.208131 / 0.419271 (-0.211141) | 0.036030 / 0.043533 (-0.007502) | 0.241551 / 0.255139 (-0.013588) | 0.260741 / 0.283200 (-0.022459) | 0.018045 / 0.141683 (-0.123638) | 1.175308 / 1.452155 (-0.276847) | 1.192160 / 1.492716 (-0.300556) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.094579 / 0.018006 (0.076573) | 0.309850 / 0.000490 (0.309360) | 0.000232 / 0.000200 (0.000032) | 0.000052 / 0.000054 (-0.000002) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.019519 / 0.037411 (-0.017892) | 0.062201 / 0.014526 (0.047675) | 0.074017 / 0.176557 (-0.102539) | 0.121987 / 0.737135 (-0.615148) | 0.078958 / 0.296338 (-0.217380) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.286306 / 0.215209 (0.071097) | 2.777004 / 2.077655 (0.699350) | 1.481445 / 1.504120 (-0.022675) | 1.348643 / 1.541195 (-0.192552) | 1.382257 / 1.468490 (-0.086234) | 0.571436 / 4.584777 (-4.013341) | 2.373279 / 3.745712 (-1.372433) | 2.749366 / 5.269862 (-2.520496) | 1.724937 / 4.565676 (-2.840739) | 0.062233 / 0.424275 (-0.362042) | 0.005013 / 0.007607 (-0.002594) | 0.339623 / 0.226044 (0.113579) | 3.385770 / 2.268929 (1.116842) | 1.832023 / 55.444624 (-53.612601) | 1.556172 / 6.876477 (-5.320305) | 1.573301 / 2.142072 (-0.568772) | 0.648866 / 4.805227 (-4.156361) | 0.121228 / 6.500664 (-6.379436) | 0.041684 / 0.075469 (-0.033786) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.974595 / 1.841788 (-0.867192) | 11.519692 / 8.074308 (3.445383) | 9.773075 / 10.191392 (-0.418317) | 0.138149 / 0.680424 (-0.542274) | 0.014068 / 0.534201 (-0.520133) | 0.288161 / 0.579283 (-0.291122) | 0.272832 / 0.434364 (-0.161532) | 0.324476 / 0.540337 (-0.215862) | 0.419962 / 1.386936 (-0.966974) |

</details>
PyArrow==latest

<details>
<summary>Show updated benchmarks!</summary>

### Benchmark: benchmark_array_xd.json

| metric | read_batch_formatted_as_numpy after write_array2d | read_batch_formatted_as_numpy after write_flattened_sequence | read_batch_formatted_as_numpy after write_nested_sequence | read_batch_unformated after write_array2d | read_batch_unformated after write_flattened_sequence | read_batch_unformated after write_nested_sequence | read_col_formatted_as_numpy after write_array2d | read_col_formatted_as_numpy after write_flattened_sequence | read_col_formatted_as_numpy after write_nested_sequence | read_col_unformated after write_array2d | read_col_unformated after write_flattened_sequence | read_col_unformated after write_nested_sequence | read_formatted_as_numpy after write_array2d | read_formatted_as_numpy after write_flattened_sequence | read_formatted_as_numpy after write_nested_sequence | read_unformated after write_array2d | read_unformated after write_flattened_sequence | read_unformated after write_nested_sequence | write_array2d | write_flattened_sequence | write_nested_sequence |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.005668 / 0.011353 (-0.005685) | 0.003637 / 0.011008 (-0.007371) | 0.049582 / 0.038508 (0.011074) | 0.030982 / 0.023109 (0.007872) | 0.273036 / 0.275898 (-0.002862) | 0.297562 / 0.323480 (-0.025918) | 0.004382 / 0.007986 (-0.003603) | 0.002763 / 0.004328 (-0.001566) | 0.050807 / 0.004250 (0.046556) | 0.046914 / 0.037052 (0.009862) | 0.287443 / 0.258489 (0.028954) | 0.319694 / 0.293841 (0.025853) | 0.051110 / 0.128546 (-0.077436) | 0.010650 / 0.075646 (-0.064997) | 0.058254 / 0.419271 (-0.361018) | 0.033419 / 0.043533 (-0.010114) | 0.275634 / 0.255139 (0.020495) | 0.288618 / 0.283200 (0.005419) | 0.018004 / 0.141683 (-0.123678) | 1.134166 / 1.452155 (-0.317989) | 1.192533 / 1.492716 (-0.300183) |

### Benchmark: benchmark_getitem\_100B.json

| metric | get_batch_of\_1024\_random_rows | get_batch_of\_1024\_rows | get_first_row | get_last_row |
|--------|---|---|---|---|
| new / old (diff) | 0.098573 / 0.018006 (0.080566) | 0.308152 / 0.000490 (0.307662) | 0.000249 / 0.000200 (0.000049) | 0.000045 / 0.000054 (-0.000010) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022443 / 0.037411 (-0.014968) | 0.075628 / 0.014526 (0.061103) | 0.088807 / 0.176557 (-0.087750) | 0.127519 / 0.737135 (-0.609617) | 0.090156 / 0.296338 (-0.206182) |

### Benchmark: benchmark_iterating.json

| metric | read 5000 | read 50000 | read_batch 50000 10 | read_batch 50000 100 | read_batch 50000 1000 | read_formatted numpy 5000 | read_formatted pandas 5000 | read_formatted tensorflow 5000 | read_formatted torch 5000 | read_formatted_batch numpy 5000 10 | read_formatted_batch numpy 5000 1000 | shuffled read 5000 | shuffled read 50000 | shuffled read_batch 50000 10 | shuffled read_batch 50000 100 | shuffled read_batch 50000 1000 | shuffled read_formatted numpy 5000 | shuffled read_formatted_batch numpy 5000 10 | shuffled read_formatted_batch numpy 5000 1000 |
|--------|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 0.294493 / 0.215209 (0.079284) | 2.862084 / 2.077655 (0.784429) | 1.585962 / 1.504120 (0.081842) | 1.466366 / 1.541195 (-0.074829) | 1.503306 / 1.468490 (0.034816) | 0.581524 / 4.584777 (-4.003253) | 2.475593 / 3.745712 (-1.270120) | 2.852014 / 5.269862 (-2.417847) | 1.834047 / 4.565676 (-2.731630) | 0.064009 / 0.424275 (-0.360266) | 0.005094 / 0.007607 (-0.002514) | 0.355960 / 0.226044 (0.129916) | 3.428849 / 2.268929 (1.159920) | 1.958501 / 55.444624 (-53.486124) | 1.675448 / 6.876477 (-5.201029) | 1.719960 / 2.142072 (-0.422113) | 0.659609 / 4.805227 (-4.145618) | 0.119036 / 6.500664 (-6.381628) | 0.041800 / 0.075469 (-0.033669) |

### Benchmark: benchmark_map_filter.json

| metric | filter | map fast-tokenizer batched | map identity | map identity batched | map no-op batched | map no-op batched numpy | map no-op batched pandas | map no-op batched pytorch | map no-op batched tensorflow |
|--------|---|---|---|---|---|---|---|---|---|
| new / old (diff) | 1.025955 / 1.841788 (-0.815833) | 12.432417 / 8.074308 (4.358108) | 10.444854 / 10.191392 (0.253462) | 0.130106 / 0.680424 (-0.550318) | 0.015655 / 0.534201 (-0.518546) | 0.288184 / 0.579283 (-0.291099) | 0.285023 / 0.434364 (-0.149340) | 0.329244 / 0.540337 (-0.211093) | 0.415484 / 1.386936 (-0.971452) |

</details>
</details>

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