Centralize existing SQLAlchemy snippets in repository layer

[LuiggiTenorioK]

As described in #142, we are going to move all the SQLAlchemy operations to the repository layer without changing any behaivor.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 94.17476% with 12 lines in your changes missing coverage. Please review.

Project coverage is 72.96%. Comparing base (e48d4dc) to head (4531c30).
Report is 4 commits behind head on main.

Files with missing lines	Patch %	Lines
autosubmit_api/bgtasks/tasks/status_updater.py	78.94%	4 Missing ⚠️
autosubmit_api/repositories/experiment_status.py	94.36%	4 Missing ⚠️
autosubmit_api/builders/experiment_builder.py	75.00%	2 Missing ⚠️
autosubmit_api/repositories/experiment_details.py	97.82%	1 Missing ⚠️
...mit_api/workers/business/process_graph_drawings.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #145      +/-   ##
==========================================
+ Coverage   72.38%   72.96%   +0.57%     
==========================================
  Files          86       89       +3     
  Lines        7765     7872     +107     
==========================================
+ Hits         5621     5744     +123     
+ Misses       2144     2128      -16     

Flag	Coverage Δ
fast-tests	72.96% <94.17%> (+0.57%)	⬆️

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