[jvm-packages] Add getNumFeature method (#6075)

* Add getNumFeature to the Java API
* Add getNumFeature to the Scala API
* Add unit tests for getNumFeature

Co-authored-by: Philip Hyunsu Cho <chohyu01@cs.washington.edu>

jvm-packages/xgboost4j/src/main/scala/ml/dmlc/xgboost4j/scala/Booster.scala

@@ -291,6 +291,14 @@ class Booster private[xgboost4j](private[xgboost4j] var booster: JBooster)
         .asScala.mapValues(_.doubleValue).toSeq: _*)
   }
 
+  /**
+    * Get the number of model features.
+    *
+    * @return number of features
+    */
+  @throws(classOf[XGBoostError])
+  def getNumFeature: Long = booster.getNumFeature
+
   def getVersion: Int = booster.getVersion
 
   def toByteArray: Array[Byte] = {