disable array reader cache for nested fields

[adriangb]

I'm not sure if this is the best approach.

While working on Parquet struct filter / projection pushdown I found this limitation.
If you try to read a struct column in both a projection and filter you get a panic that traces back to:

arrow-rs/parquet/src/arrow/array_reader/cached_array_reader.rs

Lines 342 to 344 in 39f0aad

	fn get_def_levels(&self) -> Option<&[i16]> {
	None // we don't allow nullable parent for now.
	}

(this causes an upstream panic because the child reader needs to know the nullability of the parent, I think)