[SPARK-10570][CORE] Add version info to json api

Add a new api endpoint `/api/v1/version` to retrieve various version info. This PR only adds support for finding the current spark version, however other version info such as jvm or scala versions can easily be added.

Author: Jakob Odersky <jodersky@gmail.com>

Closes #10760 from jodersky/SPARK-10570.

core/src/main/scala/org/apache/spark/status/api/v1/ApiRootResource.scala

@@ -177,6 +177,12 @@ private[v1] class ApiRootResource extends UIRootFromServletContext {
       @PathParam("attemptId") attemptId: String): EventLogDownloadResource = {
     new EventLogDownloadResource(uiRoot, appId, Some(attemptId))
   }
+
+  @Path("version")
+  def getVersion(): VersionResource = {
+    new VersionResource(uiRoot)
+  }
+
 }
 
 private[spark] object ApiRootResource {

core/src/main/scala/org/apache/spark/status/api/v1/VersionResource.scala

+/*
+ * Licensed to the Apache Software Foundation (ASF) under one or more
+ * contributor license agreements.  See the NOTICE file distributed with
+ * this work for additional information regarding copyright ownership.
+ * The ASF licenses this file to You under the Apache License, Version 2.0
+ * (the "License"); you may not use this file except in compliance with
+ * the License.  You may obtain a copy of the License at
+ *
+ *    http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package org.apache.spark.status.api.v1
+
+import javax.ws.rs._
+import javax.ws.rs.core.MediaType
+
+@Produces(Array(MediaType.APPLICATION_JSON))
+private[v1] class VersionResource(ui: UIRoot) {
+
+  @GET
+  def getVersionInfo(): VersionInfo = new VersionInfo(
+    org.apache.spark.SPARK_VERSION
+  )
+
+}

core/src/main/scala/org/apache/spark/status/api/v1/api.scala

@@ -237,3 +237,6 @@ class AccumulableInfo private[spark](
     val name: String,
     val update: Option[String],
     val value: String)
+
+class VersionInfo private[spark](
+  val spark: String)