Feature: plugins downloader

[Guts]

This PR add logic to download plugins which are referenced within profile.json files downloaded in QDT working directory.

It does not include:

• the installation/copy of downloaded plugins
• the local plugins

[codecov-commenter]

Codecov Report

Merging #168 (dc2b91f) into main (f1a18c9) will decrease coverage by 2.66%.
The diff coverage is 43.29%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #168      +/-   ##
==========================================
- Coverage   66.49%   63.84%   -2.66%     
==========================================
  Files          31       33       +2     
  Lines        1382     1557     +175     
  Branches      304      342      +38     
==========================================
+ Hits          919      994      +75     
- Misses        392      489      +97     
- Partials       71       74       +3     

Impacted Files	Coverage Δ
qgis_deployment_toolbelt/commands/cli_upgrade.py	61.90% <ø> (-0.60%)	⬇️
qgis_deployment_toolbelt/utils/file_downloader.py	30.00% <0.00%> (-2.44%)	⬇️
...ployment_toolbelt/jobs/job_plugins_synchronizer.py	20.38% <20.38%> (ø)
qgis_deployment_toolbelt/constants.py	87.09% <60.00%> (-12.91%)	⬇️
qgis_deployment_toolbelt/plugins/plugin.py	78.57% <78.57%> (ø)
qgis_deployment_toolbelt/profiles/qdt_profile.py	85.43% <80.00%> (-0.43%)	⬇️
...loyment_toolbelt/jobs/job_profiles_synchronizer.py	69.51% <85.71%> (+0.37%)	⬆️
qgis_deployment_toolbelt/cli.py	86.30% <100.00%> (+2.73%)	⬆️
qgis_deployment_toolbelt/jobs/orchestrator.py	84.84% <100.00%> (+0.47%)	⬆️

[Ducarouge]

LGTM