[coll] Implement a new tracker and a communicator. (#9650)

* [coll] Implement a new tracker and a communicator.

The new tracker and communicators communicate through the use of JSON documents. Along
with which, communicators are aware of each other.

src/collective/allgather.h

@@ -0,0 +1,23 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#pragma once
+#include <cstddef>  // for size_t
+#include <cstdint>  // for int32_t
+#include <memory>   // for shared_ptr
+
+#include "comm.h"          // for Comm, Channel
+#include "xgboost/span.h"  // for Span
+
+namespace xgboost::collective {
+namespace cpu_impl {
+/**
+ * @param worker_off Segment offset. For example, if the rank 2 worker specifis worker_off
+ *                   = 1, then it owns the third segment.
+ */
+[[nodiscard]] Result RingAllgather(Comm const& comm, common::Span<std::int8_t> data,
+                                   std::size_t segment_size, std::int32_t worker_off,
+                                   std::shared_ptr<Channel> prev_ch,
+                                   std::shared_ptr<Channel> next_ch);
+}  // namespace cpu_impl
+}  // namespace xgboost::collective