Fediverse social sign-in

[erlend-sh]

We eventually want feature parity with this:

https://zotum.net/channel/fentiger?mid=855ad721-1911-4a85-8ebd-afb73f2519fe

https://codeberg.org/FenTiger/FedIAM

Rauthy prior art:

• https://github.com/sebadob/rauthy/discussions/145
• https://github.com/sebadob/rauthy/discussions/146