Fix disable_metadata_checks behavior and fix StaticItem support of bytes content

[benoit74]

Rationale

• libzim StringProvider supports both bytes and str types as content but StaticItem accepted only str in its type hint
  • this was detected while migrating add_item_for to use new StaticItem parameters
• last PR Relax metadata check #135 was incorrect 🤦🏻
• using main code on kolibri showed an issue when the content passed to add_item_for/StaticItem is an empty string "" ; content is not set and StaticItem fails to know which content provider has to be used

Changes

• accept both bytes and str types as content in StaticItem
• accept both bytes and str types as content in scraperlib StringProvider
• check at runtime that content type is correct before passing it to the StringProvider
• reverse/fix logic of disable_metadata_checks
• always set item attributes when passed, even if 'False'/empty/whatever
• add sufficient tests, including the addition of a non-UTF8 item into the ZIM (as bytes) and its decoding

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (a25ec60) 100.00% compared to head (d180e35) 100.00%.

Additional details and impacted files

@@            Coverage Diff            @@
##              main      #136   +/-   ##
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  Coverage   100.00%   100.00%           
=========================================
  Files           32        32           
  Lines         1373      1374    +1     
  Branches       239       240    +1     
=========================================
+ Hits          1373      1374    +1     

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