refactor: use openfoodfacts.ml for object detection

[raphael0202]

• use ml submodule for object detection
• add a ModelConfig pydantic class to specify model configuration. Previously, the model configuration was not clear and could lead to bugs.
• add a threshold parameter in model predict route

[codecov]

Codecov Report

Attention: Patch coverage is 55.10204% with 22 lines in your changes missing coverage. Please review.

Project coverage is 55.41%. Comparing base (bb72dce) to head (faf4c41).
Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
robotoff/prediction/object_detection/core.py	53.84%	18 Missing ⚠️
robotoff/app/api.py	0.00%	3 Missing ⚠️
robotoff/cli/main.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1493      +/-   ##
==========================================
+ Coverage   54.94%   55.41%   +0.47%     
==========================================
  Files          85       85              
  Lines        8855     8764      -91     
==========================================
- Hits         4865     4857       -8     
+ Misses       3990     3907      -83     

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