Function extract_nested_fields 

pub fn extract_nested_fields<P>(
    datatype: &DataType,
    predicate: P,
) -> Option<Vec1<(Selector, DataType)>>
where
    P: Fn(&DataType) -> bool,

Extract nested fields from a struct array that match a predicate.

Returns None if no fields match the predicate, or if datatype is not a DataType::Struct.