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

[paulbauriegel]

I tested it a bit with a local set-up but it does not seem to work. I need some time to debug it, to understand why it is not working.

[frascuchon]

I tested it a bit with a local set-up but it does not seem to work. I need some time to debug it, to understand why it is not working.

I've pushed some missing tests to see the expected values https://github.com/argilla-io/argilla/blob/bf2a0f64a991731602cd521188dd95bb3896d7ea/argilla-server/tests/unit/api/handlers/v1/test_oauth2.py