Properly serialize trp's extra_attributes

[Xarthisius]

This pull request adds support for storing and serializing extra custom attributes on TrustedResearchPerformance objects. These extra attributes can now be passed when adding a performance, are included in the JSON-LD output, and are preserved through save/load cycles. The changes also include a new test to verify this functionality.

Support for extra attributes on performances:

• Added an extra_attributes field to the TrustedResearchPerformance model to store arbitrary custom key-value pairs.
• Updated the to_jsonld method to include any extra_attributes in the serialized output.
• Modified the deserialization logic to extract unknown (custom) keys from the JSON-LD data and store them in extra_attributes. [1] [2]
• Updated the add_performance method in the TRO model to set extra_attributes directly on the performance object.

Testing:

• Added a test (test_add_performance_extra_attrs) to ensure that extra attributes passed to add_performance are correctly included and persisted.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.78%. Comparing base (b24d0c2) to head (6b7bd10).
⚠️ Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #26      +/-   ##
==========================================
+ Coverage   94.66%   94.78%   +0.12%     
==========================================
  Files          15       15              
  Lines        1030     1035       +5     
==========================================
+ Hits          975      981       +6     
+ Misses         55       54       -1     

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