Feature: add http caching mechanism

[Guts]

This PR aims to improve plugin's performance regarding HTTP requests which are performed to retrieve remote image length:

• add a new option cache_dir
• rely on https://pypi.org/project/CacheControl/ to manage local cache requests

For now, it works only for GET requests. See upstream issue: psf/cachecontrol#337

[sonarcloud]

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]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 80.99%. Comparing base (96c0404) to head (96fec9f).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #309      +/-   ##
==========================================
+ Coverage   80.75%   80.99%   +0.23%     
==========================================
  Files          10       10              
  Lines         634      642       +8     
  Branches      131      131              
==========================================
+ Hits          512      520       +8     
  Misses         84       84              
  Partials       38       38              

Flag	Coverage Δ
unittests	80.84% <100.00%> (+0.24%)	⬆️

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Files	Coverage Δ
mkdocs_rss_plugin/config.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/constants.py	100.00% <100.00%> (ø)
mkdocs_rss_plugin/plugin.py	90.22% <100.00%> (+0.14%)	⬆️
mkdocs_rss_plugin/util.py	74.33% <100.00%> (+0.29%)	⬆️