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

[liamzwbao]

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 #9631 (comment)

[sdf-jkl]

I think #10145 will shape the framework for this one.