Support for Postgres extension types?

I'm wondering if anyone has given any thought to how a user might extend ADBC's default types.

In particular, I'm curious if it would be possible to add support for PostGIS's geospatial types (via the [geoarrow](https://github.com/geoarrow/geoarrow) encoding). I'm guessing these sort of extension types wouldn't make sense to declare as part of arrow-adbc itself; would there maybe be a way to inject other type handling into the `TypeMapping`?