refactor: split Policy interface and create PolicyGroup interface

[fabriziosestito]

Description

Fixes: #842

[codecov]

Codecov Report

Attention: Patch coverage is 95.65217% with 1 line in your changes missing coverage. Please review.

Project coverage is 70.51%. Comparing base (8237b77) to head (9b2cabd).
Report is 6 commits behind head on main.

Files with missing lines	Patch %	Lines
api/policies/v1/policygroup_validation.go	94.11%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #886      +/-   ##
==========================================
+ Coverage   70.40%   70.51%   +0.10%     
==========================================
  Files          30       30              
  Lines        2619     2601      -18     
==========================================
- Hits         1844     1834      -10     
+ Misses        602      594       -8     
  Partials      173      173              

Flag	Coverage Δ
integration-tests	60.45% <26.08%> (+0.03%)	⬆️
unit-tests	39.21% <94.11%> (+0.90%)	⬆️

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[flavio]

nice refactor!