[Fixes #12627] Include assets inside B/R

[mattiagiupponi]

• Add assets folder to the backup
• Add Asset foolder to restore
• During the restore the asset path is replaced with the new ASSETS_ROOT if needed
• Now the asset works with a custom Manager named AssetPolymorphicManager which is required to correctly handle the dump during the backup procedure. There is an hack in the polymorphic model, but works only if the dump is called via CLI
• align test and code to the new changes

Checklist

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[codecov]

Codecov Report

Attention: Patch coverage is 38.15789% with 47 lines in your changes missing coverage. Please review.

Project coverage is 64.15%. Comparing base (30b84a2) to head (acae910).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #12628      +/-   ##
==========================================
- Coverage   64.17%   64.15%   -0.02%     
==========================================
  Files         889      889              
  Lines       53717    53769      +52     
  Branches     6627     6636       +9     
==========================================
+ Hits        34473    34498      +25     
- Misses      17730    17757      +27     
  Partials     1514     1514