Add global configuration (#6414)

* Add management functions for global configuration: XGBSetGlobalConfig(), XGBGetGlobalConfig().
* Add Python interface: set_config(), get_config(), and config_context().
* Add unit tests for Python
* Add R interface: xgb.set.config(), xgb.get.config()
* Add unit tests for R

Co-authored-by: Jiaming Yuan <jm.yuan@outlook.com>

R-package/R/xgboost.R

@@ -91,6 +91,8 @@ NULL
 #' @importFrom data.table setkeyv
 #' @importFrom data.table setnames
 #' @importFrom magrittr %>%
+#' @importFrom jsonlite fromJSON
+#' @importFrom jsonlite toJSON
 #' @importFrom utils object.size str tail
 #' @importFrom stats predict
 #' @importFrom stats median