[REFACTOR][ENHANCEMENT] `argilla server`: better OAuth2 integration

[frascuchon]

Description

This PR improves the social_core integration, making setup backends simpler.

Improvements:

• Reading SOCIAL_AUTH_* environment variables
• Remove unnecessary settings attribute enabled
• Add more backends with no-code: extra_backends* (some backends may be not fully supported, but users can create issues for that)

Type of change

• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)
• Documentation update

How Has This Been Tested

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
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 89.51613% with 13 lines in your changes missing coverage. Please review.

Project coverage is 91.82%. Comparing base (f346a84) to head (0bea71e). Report is 38 commits behind head on develop.

Files with missing lines	Patch %	Lines
..._server/security/authentication/oauth2/provider.py	71.87%	9 Missing :warning:
...ver/security/authentication/oauth2/auth_backend.py	0.00%	2 Missing :warning:
...server/security/authentication/oauth2/_backends.py	97.22%	1 Missing :warning:
...illa-server/src/argilla_server/validators/users.py	95.00%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #5689 +/- ## =========================================== + Coverage 91.08% 91.82% +0.73% =========================================== Files 150 159 +9 Lines 6250 6738 +488 =========================================== + Hits 5693 6187 +494 + Misses 557 551 -6 ``` | [Flag](https://app.codecov.io/gh/argilla-io/argilla/pull/5689/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | Coverage Δ | | |---|---|---| | [argilla-server](https://app.codecov.io/gh/argilla-io/argilla/pull/5689/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io) | `91.82% <89.51%> (+0.73%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=argilla-io#carryforward-flags-in-the-pull-request-comment) to find out more.

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[github-actions[bot]]

The URL of the deployed environment for this PR is https://argilla-quickstart-pr-5689-ki24f765kq-no.a.run.app