Add Keycloak for a brokered identity provider

[EnigmaCurry]

Traefik-forward-auth is currently setup to allow either Gitea, GitHub, or Google to act as identity provider. However, only one can be used at a time.

It would be nice to let users login with their preferred identity provider, so they can login with any of them. If we install Keycloak, we can set it up with traefik-forward-auth the same way, but allow this brokered identity login to N different providers.

[EnigmaCurry]

Consider these alternatives https://github.com/lastlogin-io/obligator#comparison-is-the-thief-of-joy