refactor: backgroundTasks - around the new "upload" method

[monsieurtanuki]

Impacted files:

• abstract_background_task.dart: refactored around the upload method
• background_task_details.dart: now implements the new upload method
• background_task_image.dart: now implements the new upload method
• product_refresher.dart: removed now useless method saveAndRefresh

What

• Refactored while trying to understand - in the end, the background tasks differ only by the upload part (the rest is just the same)
• It's not just refactoring for refactoring; the idea is to make the code easier to upgrade (as we'll have to do it).

Part of

• Background + offline incoherences on multiple offline edits #3018

[AshAman999]

Everything looks good to me ,
Looks more readable now !!!

[codecov-commenter]

Codecov Report

Merging #3028 (936def0) into develop (f893237) will increase coverage by 0.02%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           develop   #3028      +/-   ##
==========================================
+ Coverage     6.52%   6.54%   +0.02%     
==========================================
  Files          244     244              
  Lines        12075   12036      -39     
==========================================
  Hits           788     788              
+ Misses       11287   11248      -39     

Impacted Files	Coverage Δ
...h_app/lib/background/abstract_background_task.dart	0.00% <0.00%> (ø)
...th_app/lib/background/background_task_details.dart	1.96% <0.00%> (+0.34%)	⬆️
...ooth_app/lib/background/background_task_image.dart	0.00% <0.00%> (ø)
...pp/lib/pages/product/common/product_refresher.dart	2.63% <ø> (+1.22%)	⬆️

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[monsieurtanuki]

Thank you @AshAman999 for your review!