Support atproto-oauth login (Bluesky)

[erlend-sh]

Since Hanko supports custom OAuth connections it’d be great to see AT protocol’s flavor of OAuth added as an option.

The big upside of atproto is that it empowers users with legitimate ownership of their web identity and social graph.

The specific case I have in mind is to support atproto logins in fika.bar, which uses Hanko.

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Reference implementation:

• ATProto support sebadob/rauthy#644
• https://github.com/streamplace/oatproxy
• https://github.com/graze-social/aip