Support XGBoost 3.1

[hcho3]

Closes #635
Closes #633

TODOs

• Support vector base_score field (multiple outputs)
• Parse cats field. Note that we will parse the field but will not store them in the Treelite model. See [FEA] Store categorical encoding in Treelite model representation #639.
• Update XGBoost integration tests to remove the use of legacy binary format, which was removed from XGBoost 3.1.
• Add test coverage, to ensure that exceptions are thrown for XGBoost models with categorical encoding.

[codecov]

Codecov Report

❌ Patch coverage is 82.57576% with 23 lines in your changes missing coverage. Please review.
✅ Project coverage is 84.33%. Comparing base (aea9a94) to head (e233818).
⚠️ Report is 13 commits behind head on mainline.

Files with missing lines	Patch %	Lines
...el_loader/detail/xgboost_json/delegated_handler.cc	77.22%	23 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##           mainline     #638      +/-   ##
============================================
- Coverage     84.78%   84.33%   -0.45%     
============================================
  Files            77       77              
  Lines          6756     6869     +113     
  Branches        531      543      +12     
============================================
+ Hits           5728     5793      +65     
- Misses         1028     1076      +48     

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