Add keycloak SSO

[paulbauriegel]

Add keycloak SSO

Based on discussion in #5691

Points that need some feedback:

• A lot of configurations are set via env variables now. Not sure if that's ideal, error messages if something is not set correctly can be rather cryptic with social auth lib
• I added the Keycloak logo to the Oauth button id the provider is keycloak, generally the same could also be done for the HF logo not having a separate button
• Is the documentation to set-up a keycloak server sufficient?

Type of change

• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested
Local build & Keycloak installation as described in the documentation.

Checklist

• I added relevant documentation
• I followed the style guidelines of this project
• I did a self-review of my code
• I made corresponding changes to the documentation
• I confirm My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• TODO I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[davidberenstein1957]

Related #5691

[frascuchon]

I think we only need this one. The rest of the required values (auth URL or access token URL) should be returned by the oicd_config endpoint based on the oidc endpoint

[paulbauriegel]

Yes this works, I updated the docs accordingly. Now configuring keycloak would work by just setting the SOCIAL_AUTH_OIDC_ENDPOINT environment variable in addition to the oauth yaml.