Ability to register adapter in-process

[cancan101]

Add an ability to register a new adapter in-process, along the lines of how sqlalchemy allows registering new dialect in-process. Currently it looks like the only way to add an adapter to shillelagh is through an entrypoint.

[betodealmeida]

Looks like it's possible to do it at runtime via pkg_resources: https://stackoverflow.com/a/48666503/807118

There's also a 3rd-party library that helps: https://prybar.readthedocs.io/en/stable/

But it might be easier to use a singleton for the registry as the source of truth, and populate it from iter_entry_points.