feat: Improve default copy method to also copy placeholders and plugins

[fsbraun]

Description

Versioning allows specifying a copy method to allow for copying foreign keys, one-to-one fields in a meaningful way for the specific model.

One of the most relevant relations will be placeholders and their plugins.

To avoid code repetition of largely identical copy methods in any package using djangocms-versioning, I propose to include coyping placeholders in versioned models by default.

This would make custom copy functions obsolete in alias and blog and other content objects to come.

Also, if the verisoned model has a method copy_relations it is called on the new object allowing to keep simple copy information within the model.

Tests follow.

Related resources

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Checklist

• I have opened this pull request against master
• I have added or modified the tests when changing logic
• I have followed the conventional commits guidelines to add meaningful information into the changelog
• I have read the contribution guidelines and I have joined #workgroup-pr-review on
  Slack to find a “pr review buddy” who is going to review my pull request.

[codecov]

Codecov Report

Attention: Patch coverage is 23.52941% with 13 lines in your changes missing coverage. Please review.

Project coverage is 90.78%. Comparing base (f90c5b2) to head (9d3ca00).
Report is 33 commits behind head on master.

Files with missing lines	Patch %	Lines
djangocms_versioning/datastructures.py	23.52%	11 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #345      +/-   ##
==========================================
- Coverage   90.88%   90.78%   -0.11%     
==========================================
  Files          72       72              
  Lines        2546     2679     +133     
  Branches      361      313      -48     
==========================================
+ Hits         2314     2432     +118     
- Misses        168      174       +6     
- Partials       64       73       +9     

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