[Variant] Consider consolidating struct builders between `variant_get` and `shred_variant`

It looks like this `shredded` param allows quite a bit of code reuse. Should we look at other shred-vs-get pairs to see if similar consolidation is possible? Struct, in particular, is likely to be complex?

_Originally posted by @scovich in https://github.com/apache/arrow-rs/pull/9631#discussion_r3015998805_

Start by locating the implementations of `variant_get` and `shred_variant`, then compare how their struct builders use the `shredded` parameter. Review other shred-versus-get pairs for similar patterns; done means the consolidation scope is resolved and the relevant builders share an appropriate implementation.

Written by the indexing model from the issue text.