refactor: Review server app setup

[frascuchon]

Description

In this PR the argilla.server.server module has been removed and all server application setup has been moved to the argilla.server.app module.

Also, all the app creation flow has been wrapped into the create_server_app function

Type of change

(Please delete options that are not relevant. Remember to title the PR according to the type of change)

• New feature (non-breaking change which adds functionality)
• Refactor (change restructuring the codebase without changing functionality)
• Improvement (change adding some improvement to an existing functionality)

How Has This Been Tested

(Please describe the tests that you ran to verify your changes. And ideally, reference tests)

Running locally `python -m argilla server start``

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
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• I filled out the contributor form (see text above)
• I have added relevant notes to the CHANGELOG.md file (See https://keepachangelog.com/)

[github-actions]

The URL of the deployed environment for this PR is https://argilla-quickstart-pr-4432-ki24f765kq-no.a.run.app

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (6630d7b) 90.13% compared to head (a1cc818) 91.32%.
Report is 552 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #4432      +/-   ##
===========================================
+ Coverage    90.13%   91.32%   +1.18%     
===========================================
  Files          233      334     +101     
  Lines        12493    19315    +6822     
===========================================
+ Hits         11261    17640    +6379     
- Misses        1232     1675     +443     

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[jfcalvo]

Maybe we should have a similar a similar function for Redis that do a ping to Redis (not here in your PR, I mean in my PR #4427) so we now early if Redis is not available, showing an error message.