Support any OIDC compatible auth server

[Powersource]

Right now we only support Keycloak as the auth server, but I think it wouldn't be too difficult to make realities work with any OIDC server. I think the issues we'd have to fix:

• Update env vars to be generic. E.g. not have one called KEYCLOAK_REALM
• The slightly bigger issue is to stop using keycloak-connect and keycloak-connect-graphql in the backend, but this could probably/hopefully be done by going back to the JWT verification stuff we had before keycloak.

Step 1 would probably be to have someone actually ask for this though :P even if you could argue that this would either way end up with cleaner code.