Cudf support. (#4745)

* Initial support for cudf integration.

* Add two C APIs for consuming data and metainfo.

* Add CopyFrom for SimpleCSRSource as a generic function to consume the data.

* Add FromDeviceColumnar for consuming device data.

* Add new MetaInfo::SetInfo for consuming label, weight etc.

tests/cpp/data/test_simple_csr_source.cc

@@ -1,10 +1,15 @@
 // Copyright by Contributors
-#include <xgboost/data.h>
+#include <gtest/gtest.h>
 #include <dmlc/filesystem.h>
+
+#include <xgboost/data.h>
+#include <xgboost/json.h>
 #include "../../../src/data/simple_csr_source.h"
 
 #include "../helpers.h"
 
+namespace xgboost {
+
 TEST(SimpleCSRSource, SaveLoadBinary) {
   dmlc::TemporaryDirectory tempdir;
   const std::string tmp_file = tempdir.path + "/simple.libsvm";
@@ -33,3 +38,4 @@ TEST(SimpleCSRSource, SaveLoadBinary) {
   delete dmat;
   delete dmat_read;
 }
+}  // namespace xgboost