Updated Variant array and the new VariantGet expression (#7877)

## Summary

This PR includes two big changes as Variant moves closer to readiness.

1. Potentially holding the `shredded` child of a variant array in the
canonical VariantArray
2. A `VariantGet` expression that can pull extract data out of variant
arrays, either in a typed way or as a more opaque `Variant`.

For reviewers, some relevant context might be:
1. The [VariantGet](vortex-data/rfcs#58) RFC:
this RFC takes some lessons I've learned working on this into account
and reflects my updated view of this problem.
2. The original [Variant
type](https://vortex-data.github.io/rfcs/rfc/0015.html) RFC

I think the Parquet spec is also a pretty good read and a very heavy
influence of this work -
[`Shredding`](https://parquet.apache.org/docs/file-format/types/variantshredding/)
and the [`Variant
type`](https://parquet.apache.org/docs/file-format/types/variantencoding/).

---------

Signed-off-by: "Adam Gutglick" <adam@spiraldb.com>
Signed-off-by: Adam Gutglick <adam@spiraldb.com>

Author: AdamGS

Cargo.lock

@@ -34,5 +34,8 @@ vortex-session = { workspace = true }
 
 [dev-dependencies]
 rstest = { workspace = true }
+tokio = { workspace = true, features = ["full"] }
 vortex-array = { workspace = true, features = ["_test-harness"] }
-
+vortex-file = { workspace = true, features = ["tokio"] }
+vortex-io = { workspace = true, features = ["tokio"] }
+vortex-layout = { workspace = true }