[ENHANCEMENT] Argilla frontend: Add default OAuth button

[frascuchon]

Description

This PR adds logic to the sign-in page to include a default OAuth provider button. So, users can sign using whatever provider is defined.

Refs #5689

Type of change

• 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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.39%. Comparing base (8700b81) to head (cee0e38).
Report is 5 commits behind head on refactor/argilla-server/better-oauth2-integration.

Additional details and impacted files

@@                                  Coverage Diff                                  @@
##           refactor/argilla-server/better-oauth2-integration    #5695      +/-   ##
=====================================================================================
+ Coverage                                              90.94%   91.39%   +0.45%     
=====================================================================================
  Files                                                    148      163      +15     
  Lines                                                   6237     6662     +425     
=====================================================================================
+ Hits                                                    5672     6089     +417     
- Misses                                                   565      573       +8     

Flag	Coverage Δ
argilla-server	91.39% <ø> (+0.45%)	⬆️

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[github-actions]

The URL of the deployed environment for this PR is https://argilla-quickstart-pr-5695-ki24f765kq-no.a.run.app