Add custom into_arrow and from_arrow implementations for extension types

[a10y]

Our extension type system allows us to model any type that can be represented by another DType plus some opaque metadata.

We currently only have one Encoding, vortex.extension, that is reused for all extension-typed arrays. This falls short for the purposes of encoding extension types that we want to decode into specific Arrow types.

For example, Arrow has a GeoArrow extension that is quite popular. We should enable the import of GeoArrow datasets into vortex, and then be able to send GeoArrow back out.

This will require us to make a few decisions

• How do we want to make a vtable/function registry available at runtime so that we can specialize into_arrow for ExtensionArray. This will require us to have some sort of global context that is writable by extension authors and accessible from compute functions
• Arrow extension arrays like GeoArrow require additional metadata, which is attached to a Field schema and not just a DataType or ArrayRef. I don't think there's currently a place in our API to return that information