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.

plugin/sycl/objective/multiclass_obj.cc

@@ -6,7 +6,6 @@
 #pragma GCC diagnostic push
 #pragma GCC diagnostic ignored "-Wtautological-constant-compare"
 #pragma GCC diagnostic ignored "-W#pragma-messages"
-#include <rabit/rabit.h>
 #pragma GCC diagnostic pop
 
 #include <vector>

plugin/sycl/objective/regression_obj.cc

@@ -9,7 +9,6 @@
 #include <xgboost/logging.h>
 #include <xgboost/objective.h>
 #pragma GCC diagnostic pop
-#include <rabit/rabit.h>
 
 #include <cmath>
 #include <memory>