Given a shredded variant field – a (value?, typed_value?) pair – try to take one path step
deeper. For a VariantPathElement::Field, if there is no typed_value at this level, if
typed_value is not a struct, or if the requested field name does not exist, traversal returns
a missing-path step (Missing or NotShredded depending on whether value exists).
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.