OAuth/OIDC social logins

[erlend-sh]

If rustypaste supported (optional) signup via social auth providers, it’d be easier to offer it as a hosted service for anyone who doesn’t want to run it by themselves.

This would also allow for indie-auth upstarts and federated identities to integrate with rustypaste.

Some prior art:
https://github.com/RomainMichau/ActixWeb_openIDConnect/
https://github.com/thomiceli/opengist (supports OAuth logins)