Given a shredded variant field – a (value?, typed_value?) pair – try to take one path step
deeper. For a VariantPathElement::Field, the step fails if there is no typed_value at this
level, or if typed_value is not a struct, or if the requested field name does not exist.
Follows the given path as far as possible through shredded variant fields. If the path ends on a
shredded field, return it directly. Otherwise, use a row shredder to follow the rest of the path
and extract the requested value on a per-row basis.