feat: add public language support

[metaforx]

• Add the IsAllowedPublicLanguage permission to public endpoints. Preview endpoints unmodified and fetch all language content regardless of language publication state.
• Fix pytest depenndencies, add missing filer, djangocms_picture (@fsbraun: should be verified if this is an isolated problem)

Fixes #1 - languages will still output all available languages but limit access on public endpoints

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 81.15%. Comparing base (bf441ad) to head (b2ab4a4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #27      +/-   ##
==========================================
- Coverage   81.22%   81.15%   -0.07%     
==========================================
  Files          12       12              
  Lines         474      483       +9     
  Branches       41       42       +1     
==========================================
+ Hits          385      392       +7     
- Misses         63       64       +1     
- Partials       26       27       +1     

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[fsbraun]

@metaforx LGTM! Why do we need filer and/or picture in the test dependencies? I agree it probably makes sense to test with some real life plugins, though.

[metaforx]

@fsbraun:

In tests app settings.py filer is present. Git history tells me it was always present. Because they were not installed, my tests failed (migration). Should we remove all but text?

    'djangocms_link',
    'djangocms_picture',
    'djangocms_text',
    'filer',
    'easy_thumbnails',
    'tests.test_app',
    ```
    
But eventually, it would be a good idea to test all supported Django CMS supported plugins for rest compatibility.

[metaforx]

@fsbraun:

In tests app settings.py filer is present. Git history tells me it was always present. Because they were not installed, my tests failed (migration). Should we remove all but text?

    'djangocms_link',
    'djangocms_picture',
    'djangocms_text',
    'filer',
    'easy_thumbnails',
    'tests.test_app',
    ```
    
But eventually, it would be a good idea to test all supported Django CMS supported plugins for rest compatibility.

I just realized, that filer is not installed via base.txt in requirements. so i will remove it. along with easy_thumbnails.

Ok to do this in this PR?