feat: Implement new xgboost

[gandola]

Adds a new OpenML provider, openml-xgboost, built directly on the upstream XGBoost JVM binding (ml.dmlc:xgboost4j_2.12:3.4.0) to be a replacement of the H2O-embedded XGBoost.

Phase 1 covers:

• Model import — loadModel / loadSchema / validateForLoad.
• Real-time single-instance scoring — classify / getClassDistribution via Booster.inplace_predict.
• In-process (non-Spark) training — fit via XGBoost.train.

Key classes

• XgboostModelProvider — TrainingMachineLearningProvider, algorithm XGBOOST_BINARY_CLASSIFIER.
• XgboostModelCreator — loadModel / fit / validateForLoad / validateForFit.
• XgboostClassificationModel — scoring; native handle guarded by a synchronized block (the Booster handle is not thread-safe).
• XgboostSchemaUtils — shared feature-row builder used by both training and scoring (guarantees identical encoding).

Native runtime

• xgboost4j bundles native libs for linux/{x86_64,aarch64}, macos/{x86_64,aarch64}, windows/x86_64 — ARM/Graviton works out of the box.
• Requires OpenMP (libgomp/libomp) at runtime.
• No musl/Alpine native binary is published — native-dependent tests are Assume-guarded and skip gracefully there.

Dependency handling (Kryo)

xgboost4j_2.12 transitively pulls Kryo 5, whose com.esotericsoftware.kryo.Kryo collides on the classpath with the Kryo 4 (kryo-shaded) used by Spark/consumers (Kryo 5 removed the nested Kryo.DefaultInstantiatorStrategy). Since Booster implements com.esotericsoftware.kryo.KryoSerializable, kryo must still be
present at class-load. Resolution: exclude kryo from the xgboost4j dependency and re-declare it in provided scope — available for this module's own compile/tests, not propagated transitively to consumers (which supply their own Kryo).

Testing

• 20 tests; native-dependent ones Assume-skip where the native lib can't load (musl).
• Coverage: 154/159 lines, 38/40 branches. Remaining gaps are the jackson-backed loadSchema one-line delegation and two native-exception catch blocks.
• Verified train + save/reload produces identical scores; validation rejects string/free-text feature fields.

[codecov]

Codecov Report

❌ Patch coverage is 98.42932% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 83.38%. Comparing base (18a41e9) to head (511bc6f).

Files with missing lines	Patch %	Lines
...l/provider/xgboost/XgboostClassificationModel.java	95.00%	2 Missing ⚠️
...i/openml/provider/xgboost/XgboostModelCreator.java	99.07%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##             master     #156      +/-   ##
============================================
+ Coverage     81.76%   83.38%   +1.62%     
- Complexity      525      589      +64     
============================================
  Files            52       58       +6     
  Lines          1771     1962     +191     
  Branches        179      195      +16     
============================================
+ Hits           1448     1636     +188     
- Misses          234      237       +3     
  Partials         89       89              

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[jpleitao]

Overall the code looks good, just added some comments regarding (i) the set of parameters being exposed, (ii) default values for some parameters, and (iii) validations

[jpleitao]

All comments have been address, nothing further to point out (don't seem to be able to resolve threads, so leaving them open but the PR is fine by me)