Add Bluesky support

[fluffy-critter]

Bluesky is getting pretty popular and now supports OAuth.

My understanding is that there's no need for client registration, it just does something IndieAuth-esque where the client_id is a link to a metadata document. This document would probably be best configured as a URL that is then filled out by the site operator, rather than trying to shoehorn it into Authl itself, although in the long term it would be worth investigating a flow for allowing handlers to provide sideband data in some framework-agnostic way.

There's also a butt-ton of Python bsky client libraries in various states of support, although they all seem to use the legacy username/password auth flow rather than OAuth, which is not appropriate for Authl.