Map type

[lukekim]

We (Spice AI) need Map type support through the DataFusion integration.

This is start a discussion on Map type in general, as there are a number of options:

First-class DType

Does this just duplicate existing types/implementation?

List/Struct Conversion

Example of basic List/Struct Conversion implementation: https://github.com/spiceai/vortex/pull/21/changes

List/Struct could be considered as stop gap for Variant support.

An extension array

Pros/cons?