Parquet StatisticsConverter does not work for struct columns

**Describe the bug**

The `StatisticsConverter` produces all-null columns for struct-type fields.

**To Reproduce**

```rs
#[cfg(test)]
mod test_geoparquet {
use std::sync::Arc;

use arrow::array::AsArray;
use arrow::datatypes::Float32Type;
use object_store::aws::AmazonS3Builder;
use parquet::arrow::arrow_reader::ArrowReaderMetadata;
use parquet::arrow::async_reader::ParquetObjectReader;

use super::*;

#[tokio::test]
async fn test_struct_geoparquet() {
let store = Arc::new(
AmazonS3Builder::new()
.with_bucket_name("overturemaps-us-west-2")
.with_skip_signature(true)
.with_region("us-west-2")
.build()
.unwrap(),
);
let path = "release/2025-02-19.0/theme=addresses/type=address/part-00010-e084a2d7-fea9-41e5-a56f-e638a3307547-c000.zstd.parquet";
let mut object_reader = ParquetObjectReader::new(store, path.into());
let meta = ArrowReaderMetadata::load_async(&mut object_reader, Default::default())
.await
.unwrap();

let parquet_schema = meta.parquet_schema();
let column_desc = parquet_schema.column(2);

let min_bytes = meta
.metadata()
.row_group(0)
.column(2)
.statistics()
.unwrap()
.min_bytes_opt()
.unwrap();

let statistics_value_direct = f32::from_le_bytes(min_bytes.try_into().unwrap());

let converter =
StatisticsConverter::try_new("bbox", meta.schema(), meta.parquet_schema()).unwrap();
let mins = converter
.row_group_mins(meta.metadata().row_groups())
.unwrap();
let mins_struct = mins.as_struct();
let minx_bbox_minx = mins_struct.column(0).as_primitive::();

// This line fails
assert!(minx_bbox_minx.is_valid(0));
let statistics_value_via_converter = minx_bbox_minx.value(0);

assert_eq!(statistics_value_direct, statistics_value_via_converter)
}
}
```

**Expected behavior**

Some way to handle struct columns in `StatisticsConverter`.

**Additional context**

It looks like this issue has been documented in datafusion, before the `StatisticsConverter` was moved into `parquet` core.

- https://github.com/apache/datafusion/issues/10609
- https://github.com/apache/datafusion/issues/8334

It doesn't look like there was previously an issue in this repo for this.

Start with the StatisticsConverter entry point and the test_struct_geoparquet reproducer shown in the issue. Run the test against the provided Parquet metadata and trace how struct columns produce row-group minima; done means the struct field is valid and its converted minimum matches the direct statistic.

Written by the indexing model from the issue text.