Support row group pruning for struct field predicates #20871

Related to the work on struct array handling:

• Project only accessed struct leaves in Parquet row filter pushdown #20854
• Allow filters on struct fields to be pushed down into Parquet scan #20822
• Add benchmark for struct field filter pushdown in Parquet #20829

When filtering on struct fields (e.g. WHERE s['value'] > 5), Datafusion currently can not prune row groups using Parquet column statistics, even though the underlying leaf columns have valid min/max statistics stored in the parquet metadata

The issue is in the pruning predicate system. When it encounters a GetField expr like GetField(Column("s"), "value"), the column extraction logic only sees the parent struct Column(s) and doesn't resolve through to the nested field

Fixing this would mean teaching the pruning system to resolve GetField expressions down to their leaf columns, then look up the corresponding Parquet column stats. Note, the stats themselves are already there in the Parquet metadata, they're just never consulted for nested field access

On tables with many row groups, this could significantly reduce the amount of data read for struct field predicates