Adding Spark example to Write Pojo classes to Stream (apache#5470)

### Motivation

Added the class SensorReading and class ProducerSparkWithPojo  to showcase writing Pojo as Json Object to Pulsar Queue 

### Modifications

Based on the documentation written example class to write Pojos to Pulsar 
This might be more real life examples for application than bytes stream

examples/spark/src/main/java/org/apache/spark/streaming/receiver/example/ProducerSparkWithPojo.java

@@ -0,0 +1,61 @@
+/**
+ * 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.streaming.receiver.example;
+
+import java.util.concurrent.TimeUnit;
+
+import org.apache.pulsar.client.api.Producer;
+import org.apache.pulsar.client.api.PulsarClient;
+import org.apache.pulsar.client.impl.schema.JSONSchema;
+
+
+/**
+ * producer data to spark streaming receiver with Json/Pojo Object
+ *
+ * <p>Example usage:
+ *   pulsar://localhost:6650 test_src
+ */
+public class ProducerSparkWithPojo {
+
+
+	public static void main(String[] args) throws Exception {
+		if (args.length < 2) {
+			System.err.println("Missing parameters!");
+			System.err.println("Usage: <pulsar-service-url> <topic>");
+			return;
+		}
+
+		System.out.println("Parameters:");
+		System.out.println("\tServiceUrl:\t" + args[0]);
+		System.out.println("\tTopic:\t" + args[1]);
+
+		try (PulsarClient client = PulsarClient.builder().serviceUrl(args[0]).build()) {
+			try (Producer<SensorReading> producer = client.newProducer(JSONSchema.of(SensorReading.class))
+					.topic(args[1]).sendTimeout(3, TimeUnit.SECONDS).create();) {
+				for (int i = 0; i < 100; i++) {
+					SensorReading rd = new SensorReading(i, "message " + i);
+					producer.send(rd);
+				}
+			}
+		}
+
+		System.out.println("producer spark streaming msg end ...");
+	}
+
+}

examples/spark/src/main/java/org/apache/spark/streaming/receiver/example/SensorReading.java

@@ -0,0 +1,53 @@
+/**
+ * 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.streaming.receiver.example;
+
+/**
+ * Rest Pojo class to send to Producer
+ *
+ */
+public class SensorReading {
+
+	public SensorReading(int sensor_id, String sensor_value) {
+		super();
+		this.sensor_id = sensor_id;
+		this.sensor_value = sensor_value;
+	}
+
+	public int getSensor_id() {
+		return sensor_id;
+	}
+
+	public void setSensor_id(int sensor_id) {
+		this.sensor_id = sensor_id;
+	}
+
+	public String getSensor_value() {
+		return sensor_value;
+	}
+
+	public void setSensor_value(String sensor_value) {
+		this.sensor_value = sensor_value;
+	}
+
+	public int sensor_id;
+
+	public String sensor_value;
+
+}