[Feature] Add support for custom taxonomies to FacetsManager

[titouanmathis]

🔗 Linked issue

#42

❓ Type of change

• 📖 Documentation (updates to the documentation, readme or JSdoc annotations)
• 🐞 Bug fix (a non-breaking change that fixes an issue)
• 👌 Enhancement (improving an existing functionality like performance)
• ✨ New feature (a non-breaking change that adds functionality)
• 🧹 Chore (updates to the build process or auxiliary tools and libraries)
• ⚠️ Breaking change (fix or feature that would cause existing functionality to change)

📚 Description

This PR adds support for custom taxonomy to the URL parameters for the FacetsManager.

With a custom taxonomy named text_taxonomy, the following parameters can be used:

?facets[test_taxonomy]=slug1,slug2
?facets[test_taxonomy]=1,2,3
?facets[test_taxonomy__not_in]=excluded-slug
?facets[test_taxonomy__and]=slug1,slug2
?facets[test_taxonomy__exists]=1

As of now the relation operator for tax queries is not configurable and will always be AND. Usage will tell us if customization is needed.

📝 Checklist

• I have linked an issue or discussion.
• I have added tests (if possible).
• I have updated the documentation accordingly.
• I have updated the changelog.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 68.85%. Comparing base (9c5eb43) to head (2b11b89).
⚠️ Report is 4 commits behind head on develop.

Additional details and impacted files

@@              Coverage Diff              @@
##             develop      #43      +/-   ##
=============================================
+ Coverage      66.56%   68.85%   +2.29%     
- Complexity       209      222      +13     
=============================================
  Files             15       15              
  Lines            625      671      +46     
=============================================
+ Hits             416      462      +46     
  Misses           209      209              

Flag	Coverage Δ
unittests	68.85% <100.00%> (+2.29%)	⬆️

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