VTable Normalization

[gatesn]

Vortex has a very pluggable architecture that has gone through a lot of experiments. I think we're getting close to "good" but we're not quite there yet.

We currently support runtime plugins for:

• Arrays
• Layouts
• Scalar functions

And we want to add at least:

• Extension DTypes
• Aggregate Functions

We use the VortexSession object to hold a Registry of these objects, and in order to provide users with a nice strongly-typed API we have the idea of non-object-safe VTable traits. Each of these traits works slightly differently however. This discussion is to track the one true VTable structure that works for all of our extension points!

This should help address issues like #5926
However of course comes with large Rust API breaks.