Add a Map DType

[doki23]

I notice that the map type does not appear in the Logical Types, do we have some plan of it?

[blaginin]

Our medium term plan is to support a Variant DType, allowing for arbitrary JSON-like nested structures with efficient shredding of common columns.

For now, it's possible to express Map as List<Struct<String, V>>

[lukekim]

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

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?

[joseph-isaacs]

This does seem like it should be an extension DType.

We are currently in the process of making these easy to extend. @gatesn