refactor: move response_content into backend code

[nejch]

Needed in order to later add other backends that handle streaming responses differently. Just a first step, later maybe we could get rid of response_content completely from individual methods and just handle this in the request code 🤔

[codecov-commenter]

Codecov Report

Merging #2488 (24ba442) into main (07d03dc) will increase coverage by 4.27%.
The diff coverage is 87.17%.

❗ Current head 24ba442 differs from pull request most recent head ab089fb. Consider uploading reports for the commit ab089fb to get more accurate results

@@            Coverage Diff             @@
##             main    #2488      +/-   ##
==========================================
+ Coverage   91.82%   96.10%   +4.27%     
==========================================
  Files          87       87              
  Lines        5665     5667       +2     
==========================================
+ Hits         5202     5446     +244     
+ Misses        463      221     -242     

Flag	Coverage Δ
api_func_v4	82.47% <71.79%> (?)
cli_func_v4	82.93% <48.71%> (-0.05%)	⬇️
unit	87.55% <69.23%> (-0.04%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
gitlab/v4/objects/jobs.py	77.94% <33.33%> (-0.32%)	⬇️
gitlab/utils.py	95.00% <50.00%> (-2.19%)	⬇️
gitlab/_backends/protocol.py	80.00% <80.00%> (-1.25%)	⬇️
gitlab/_backends/requests_backend.py	96.92% <100.00%> (+0.92%)	⬆️
gitlab/mixins.py	92.05% <100.00%> (+5.47%)	⬆️
gitlab/v4/objects/artifacts.py	100.00% <100.00%> (ø)
gitlab/v4/objects/files.py	100.00% <100.00%> (+36.70%)	⬆️
gitlab/v4/objects/packages.py	96.22% <100.00%> (-0.07%)	⬇️
gitlab/v4/objects/projects.py	98.86% <100.00%> (ø)
gitlab/v4/objects/repositories.py	83.56% <100.00%> (+27.39%)	⬆️
... and 2 more

... and 32 files with indirect coverage changes