[Coll] Implement get host address in libxgboost. (#9644)

- Port `xgboost.tracker.get_host_ip` in C++.

tests/cpp/collective/test_tracker.cc

@@ -0,0 +1,18 @@
+/**
+ * Copyright 2023, XGBoost Contributors
+ */
+#include "../../../src/collective/tracker.h"  // for GetHostAddress
+#include "net_test.h"                         // for SocketTest
+
+namespace xgboost::collective {
+namespace {
+class TrackerTest : public SocketTest {};
+}  // namespace
+
+TEST_F(TrackerTest, GetHostAddress) {
+  std::string host;
+  auto rc = GetHostAddress(&host);
+  ASSERT_TRUE(rc.OK());
+  ASSERT_TRUE(host.find("127.") == std::string::npos);
+}
+}  // namespace xgboost::collective