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ARROW-8733: [C++][Dataset][Python] Expose RowGroupInfo statistics values

```python
stats = parquet_fragment.row_groups[0].statistics
assert stats == {
  'normal_column': {'min': 1, 'max': 2},
  'all_null_column': {'min': None, 'max': None},
  'column_without_stats': None,
}
```

Closes #7546 from bkietz/8733-row-group-statistics

Authored-by: Benjamin Kietzman <bengilgit@gmail.com>
Signed-off-by: Benjamin Kietzman <bengilgit@gmail.com>