[R] Add missing DMatrix functions (#9929)

* `XGDMatrixGetQuantileCut`
* `XGDMatrixNumNonMissing`
* `XGDMatrixGetDataAsCSR`

---------

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

R-package/NAMESPACE

@@ -37,6 +37,9 @@ export(xgb.create.features)
 export(xgb.cv)
 export(xgb.dump)
 export(xgb.gblinear.history)
+export(xgb.get.DMatrix.data)
+export(xgb.get.DMatrix.num.non.missing)
+export(xgb.get.DMatrix.qcut)
 export(xgb.get.config)
 export(xgb.ggplot.deepness)
 export(xgb.ggplot.importance)
@@ -60,6 +63,7 @@ export(xgb.unserialize)
 export(xgboost)
 import(methods)
 importClassesFrom(Matrix,dgCMatrix)
+importClassesFrom(Matrix,dgRMatrix)
 importClassesFrom(Matrix,dgeMatrix)
 importFrom(Matrix,colSums)
 importFrom(Matrix,sparse.model.matrix)
@@ -83,6 +87,7 @@ importFrom(graphics,points)
 importFrom(graphics,title)
 importFrom(jsonlite,fromJSON)
 importFrom(jsonlite,toJSON)
+importFrom(methods,new)
 importFrom(stats,median)
 importFrom(stats,predict)
 importFrom(utils,head)