feat(parquet-datasource): support virtual columns (e.g. row number) in the parquet opener

[JanKaul]

Which issue does this PR close?

Rationale for this change

The upstream parquet crate exposes "virtual columns" — columns that aren't physically stored in the file but are materialized by the reader (e.g. the per-row RowNumber extension type). DataFusion's parquet datasource didn't wire this through: a user who included a RowNumber-tagged field in their file schema would get a "column not found in parquet schema" error or a misaligned projection mask, because the opener fed the full Arrow schema to ArrowReaderOptions::with_schema (which expects real fields only) and built ProjectionMask::roots from indices that included virtual fields with no parquet leaves.

What changes are included in this PR?

• datafusion/datasource-parquet/src/opener/mod.rs
  • Add split_virtual_fields helper that partitions a schema into (real-only schema, virtual field list).
  • In PreparedParquetOpen::load, extract virtuals from logical_file_schema and pass them to ArrowReaderOptions::with_virtual_columns; store the list on MetadataLoadedParquetOpen so later stages can re-supply the real-only schema.
  • In MetadataLoadedParquetOpen, strip virtuals before each options.with_schema(...) call via a resupply_schema closure (the underlying API rejects virtuals).
  • Drop a now-unneeded #[cfg(feature = "parquet_encryption")] let mut options = options; shadow — the new unconditional with_virtual_columns write site means options always has a potential mutation, so the cfg-gated shadow is no longer required to silence unused_mut.
• datafusion/datasource-parquet/src/row_filter.rs
  • In build_projection_read_plan, filter virtual roots out of the indices passed to ProjectionMask::roots / leaf_indices_for_roots (virtuals have no parquet column to mask).
  • Add a comment on build_parquet_read_plan documenting why no symmetric filter is needed: leaf_indices_for_roots silently drops indices with no matching leaf, and the decoder appends virtuals to every batch regardless of the mask, so projected_schema stays aligned.

Are these changes tested?

Yes. Added two tests in opener/mod.rs:

• test_virtual_row_number_column — round-trips a [Int32, RowNumber(Int64)] schema across two row groups and asserts row_num == [0..6).
• test_virtual_row_number_column_with_row_group_pruning — same setup with a predicate (a >= 20) that prunes the first row group and confirms the row numbers come back as [3, 4, 5] (i.e. the virtual column reflects the file's absolute row indices, not the post-pruning sequence).

Are there any user-facing changes?

Yes — purely additive. A Field tagged with the RowNumber extension type (or any other parquet virtual-column extension) in a file schema is now honored by the parquet datasource. No existing API signatures change.