improve(downloader): use a custom HTTP transport adapter instead of rough injection

[Guts]

This PR follows up:

• feature(downloader): add option to disable SSL verification #544
• Feature: network use native system stores #444

It improve how system's store certificates are used, preferring a custom HTTP adapter to the SSL injection.

It's mainly inspired from https://stackoverflow.com/a/78265028/2556577.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.

Project coverage is 70.55%. Comparing base (e845838) to head (6294252).
Report is 2 commits behind head on main.

Files with missing lines	Patch %	Lines
qgis_deployment_toolbelt/utils/file_downloader.py	44.44%	4 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #549      +/-   ##
==========================================
- Coverage   70.55%   70.55%   -0.01%     
==========================================
  Files          47       47              
  Lines        3118     3124       +6     
  Branches      669      669              
==========================================
+ Hits         2200     2204       +4     
- Misses        724      726       +2     
  Partials      194      194              

Flag	Coverage Δ
unittests	69.97% <44.44%> (-0.01%)	⬇️

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

Files with missing lines	Coverage Δ
qgis_deployment_toolbelt/utils/file_downloader.py	72.13% <44.44%> (-0.60%)	⬇️