Revamp the rabit implementation. (#10112)

This PR replaces the original RABIT implementation with a new one, which has already been partially merged into XGBoost. The new one features:
- Federated learning for both CPU and GPU.
- NCCL.
- More data types.
- A unified interface for all the underlying implementations.
- Improved timeout handling for both tracker and workers.
- Exhausted tests with metrics (fixed a couple of bugs along the way).
- A reusable tracker for Python and JVM packages.

jvm-packages/xgboost4j/src/test/java/ml/dmlc/xgboost4j/java/DMatrixTest.java

@@ -1,5 +1,5 @@
 /*
- Copyright (c) 2014-2022 by Contributors
+ Copyright (c) 2014-2024 by Contributors
 
  Licensed under the Apache License, Version 2.0 (the "License");
  you may not use this file except in compliance with the License.
@@ -298,7 +298,7 @@ public class DMatrixTest {
 
   @Test
   public void testTrainWithDenseMatrixRef() throws XGBoostError {
-    Map<String, String> rabitEnv = new HashMap<>();
+    Map<String, Object> rabitEnv = new HashMap<>();
     rabitEnv.put("DMLC_TASK_ID", "0");
     Communicator.init(rabitEnv);
     DMatrix trainMat = null;