kindergarten-garden: add starter implementation (#1020)

* kindergarten-garden: add starter implementation

* kindergarten-garden: add constructors to starter

exercises/kindergarten-garden/src/main/java/KindergartenGarden.java

+import java.util.List;
+
+class KindergartenGarden {
+
+    KindergartenGarden(String garden, String[] students) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    KindergartenGarden(String garden) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+    List<Plant> getPlantsOfStudent(String student) {
+        throw new UnsupportedOperationException("Delete this statement and write your own implementation.");
+    }
+
+}