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.

cmake/Utils.cmake

@@ -151,6 +151,7 @@ function(xgboost_set_cuda_flags target)
   target_include_directories(
     ${target} PRIVATE
     ${xgboost_SOURCE_DIR}/gputreeshap
+    ${xgboost_SOURCE_DIR}/rabit/include
     ${CUDAToolkit_INCLUDE_DIRS})
 
   if(MSVC)