Enable setting config as default when push_to_hub

Fix #6497.

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This is ready for review @huggingface/datasets. 

Also what if the config is being overwritten and it was the default config and the user doesn't pass `set_default` ?
I'd expect the config to keep being the default one but lmk what you think

How can you unset a config as the default one? In the case you mentioned, I would expect the config not being the default one.

Maybe by passing `set_default=False` ? (set_default can be None by default)

I think that way we are unnecessarily complicating the logic of `push_to_hub` and as I told you, I would expect the contrary: the result of calling `push_to_hub` with a determined set of arguments should always be the same, independently of previous calls and the current state of the config on the Hub. Push to hub should be somehow stateless in that sense, and IMO the user expects that the push overwrites previous config if already present on the Hub. I find very confusing making it to partially update the config on the Hub.

That makes sense, having it stateless is simpler and no need to do something too fancy indeed

<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) | 0.005329 / 0.011353 (-0.006024) | 0.002998 / 0.011008 (-0.008010) | 0.063756 / 0.038508 (0.025248) | 0.051713 / 0.023109 (0.028603) | 0.248135 / 0.275898 (-0.027763) | 0.269136 / 0.323480 (-0.054344) | 0.002970 / 0.007986 (-0.005015) | 0.002566 / 0.004328 (-0.001763) | 0.048110 / 0.004250 (0.043859) | 0.038415 / 0.037052 (0.001363) | 0.254012 / 0.258489 (-0.004477) | 0.281915 / 0.293841 (-0.011926) | 0.027503 / 0.128546 (-0.101043) | 0.010370 / 0.075646 (-0.065276) | 0.208965 / 0.419271 (-0.210306) | 0.035508 / 0.043533 (-0.008024) | 0.249116 / 0.255139 (-0.006023) | 0.266350 / 0.283200 (-0.016850) | 0.018440 / 0.141683 (-0.123243) | 1.101089 / 1.452155 (-0.351066) | 1.164870 / 1.492716 (-0.327847) |

### 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) | 0.090909 / 0.018006 (0.072903) | 0.298041 / 0.000490 (0.297551) | 0.000211 / 0.000200 (0.000012) | 0.000051 / 0.000054 (-0.000004) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.018137 / 0.037411 (-0.019275) | 0.059574 / 0.014526 (0.045048) | 0.071754 / 0.176557 (-0.104803) | 0.117980 / 0.737135 (-0.619155) | 0.072903 / 0.296338 (-0.223435) |

### 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) | 0.282844 / 0.215209 (0.067635) | 2.740916 / 2.077655 (0.663261) | 1.444546 / 1.504120 (-0.059574) | 1.321904 / 1.541195 (-0.219291) | 1.356957 / 1.468490 (-0.111533) | 0.568389 / 4.584777 (-4.016388) | 2.354042 / 3.745712 (-1.391671) | 2.719427 / 5.269862 (-2.550435) | 1.719616 / 4.565676 (-2.846061) | 0.062537 / 0.424275 (-0.361738) | 0.004915 / 0.007607 (-0.002692) | 0.334716 / 0.226044 (0.108672) | 3.299499 / 2.268929 (1.030571) | 1.814629 / 55.444624 (-53.629996) | 1.515245 / 6.876477 (-5.361232) | 1.553085 / 2.142072 (-0.588987) | 0.643859 / 4.805227 (-4.161368) | 0.116650 / 6.500664 (-6.384014) | 0.041432 / 0.075469 (-0.034037) |

### 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) | 0.948227 / 1.841788 (-0.893561) | 11.331103 / 8.074308 (3.256795) | 10.209658 / 10.191392 (0.018266) | 0.126721 / 0.680424 (-0.553703) | 0.013638 / 0.534201 (-0.520563) | 0.282540 / 0.579283 (-0.296743) | 0.262635 / 0.434364 (-0.171729) | 0.335357 / 0.540337 (-0.204981) | 0.441798 / 1.386936 (-0.945138) |

</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) | 0.005200 / 0.011353 (-0.006153) | 0.003012 / 0.011008 (-0.007996) | 0.047571 / 0.038508 (0.009063) | 0.055069 / 0.023109 (0.031959) | 0.271150 / 0.275898 (-0.004748) | 0.294957 / 0.323480 (-0.028523) | 0.003922 / 0.007986 (-0.004064) | 0.002627 / 0.004328 (-0.001702) | 0.047777 / 0.004250 (0.043527) | 0.039507 / 0.037052 (0.002454) | 0.276314 / 0.258489 (0.017825) | 0.300436 / 0.293841 (0.006595) | 0.028951 / 0.128546 (-0.099595) | 0.010583 / 0.075646 (-0.065063) | 0.056535 / 0.419271 (-0.362737) | 0.032654 / 0.043533 (-0.010879) | 0.272945 / 0.255139 (0.017806) | 0.291909 / 0.283200 (0.008709) | 0.017545 / 0.141683 (-0.124138) | 1.195897 / 1.452155 (-0.256258) | 1.171855 / 1.492716 (-0.320861) |

### 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) | 0.091919 / 0.018006 (0.073913) | 0.299297 / 0.000490 (0.298807) | 0.000225 / 0.000200 (0.000025) | 0.000051 / 0.000054 (-0.000003) |

### Benchmark: benchmark_indices_mapping.json

| metric | select | shard | shuffle | sort | train_test_split |
|--------|---|---|---|---|---|
| new / old (diff) | 0.022271 / 0.037411 (-0.015140) | 0.068903 / 0.014526 (0.054377) | 0.083767 / 0.176557 (-0.092790) | 0.120239 / 0.737135 (-0.616896) | 0.083448 / 0.296338 (-0.212891) |

### 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) | 0.295353 / 0.215209 (0.080144) | 2.911452 / 2.077655 (0.833798) | 1.577941 / 1.504120 (0.073821) | 1.454514 / 1.541195 (-0.086681) | 1.459575 / 1.468490 (-0.008915) | 0.572475 / 4.584777 (-4.012302) | 2.443634 / 3.745712 (-1.302078) | 2.801171 / 5.269862 (-2.468691) | 1.724214 / 4.565676 (-2.841462) | 0.063539 / 0.424275 (-0.360736) | 0.004939 / 0.007607 (-0.002668) | 0.347705 / 0.226044 (0.121660) | 3.489591 / 2.268929 (1.220663) | 1.944952 / 55.444624 (-53.499672) | 1.652810 / 6.876477 (-5.223667) | 1.656361 / 2.142072 (-0.485712) | 0.647052 / 4.805227 (-4.158176) | 0.117286 / 6.500664 (-6.383379) | 0.040979 / 0.075469 (-0.034490) |

### 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) | 0.971761 / 1.841788 (-0.870027) | 11.770547 / 8.074308 (3.696239) | 10.402502 / 10.191392 (0.211110) | 0.128280 / 0.680424 (-0.552144) | 0.015160 / 0.534201 (-0.519041) | 0.286706 / 0.579283 (-0.292578) | 0.274539 / 0.434364 (-0.159825) | 0.324591 / 0.540337 (-0.215747) | 0.573846 / 1.386936 (-0.813090) |

</details>
</details>

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