Merging this PR will not alter performance

⚡ 2 improved benchmarks
❌ 1 regressed benchmark
✅ 1580 untouched benchmarks

Performance Changes

_{Comparing adamg/variant-json-handling (8318ee1) with develop (71319b7)}

Do these functions mix DType (JSON) with encoding (ParquetVariant)? At least the docs seem to suggest that they do.

We probably want vortex-json to expose to_json and from_json functions (or maybe this just a cast kernel) and then implement them using execute_parent for ParquetVariant.

But it seems like you are actually defining a more general DType::JSON -> DType::Variant (any variant encoding) function? In which case, it cannot be a cast because you want the shredding options, but the definition should probably still live in the vortex-json crate, and implement general-purpose shredding (per the canonical Variant array), without touching the non-shredded data?

@gatesn I've really tried to keep expressions that can't be evaluated directly from the core. I thought execute_parent rules were less flexible but I guess I can try and do that.

I think I addressed the comments. I've also made the PR a bit smaller and more focused, just on json_to_variant which I think is the more interesting code path.