Read single struct fields in ParquetOpener

[adriangb]

Currently if you have a filter or projection such as select struct_col[field] from t it gets pushed down into ParquetOpener but:

• ParquetOpener explicitly rejects filters referencing struct fields:

  datafusion/datafusion/datasource-parquet/src/row_filter.rs

  Lines 62 to 65 in c63ca33

  	//! List-aware predicates (for example, `array_has`, `array_has_all`, and
  	//! `array_has_any`) can be evaluated directly during Parquet decoding. Struct
  	//! columns and other nested projections that are not explicitly supported will
  	//! continue to be evaluated after the batches are materialized.

• Projections read the entire column instead of the single leaf column (no special handling)

I tried to fix these in ParquetOpener and found that we'd get errors from arrow-rs because of apache/arrow-rs#9066.