[NSE-1007] Add a test guide for columnar UDF

tests/udf/README.md

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+
+## Hive UDF Test Guide
+
+Currently, Gazelle supports a UDF called URLDecoder whose original implementation is under `src/`.
+To test a UDF, we should compile the original hive UDF code into a jar and put it into spark classpath.
+
+Take URLDecoder as example.
+
+1. Compile original Hive UDF code with `hive-exec` dependency provided.
+
+2. Deploy the jar and configure it into Spark classpath. We can deploy it with Gazelle jar and make
+   the below configuration to include them in spark classpath.
+
+   ```
+   --conf spark.driver.extraClassPath="YOUR_JAR_PATH"
+   --conf spark.executor.extraClassPath="YOUR_JAR_PATH"
+   ```
+
+3. Register the UDF to Spark.
+
+   ```
+   spark.sql("CREATE TEMPORARY FUNCTION URLDecoder AS 'com.intel.oap.URLDecoderNew';")
+   ```
+
+4. Run SQL with UrlDecoder used.
+
+   ```
+   spark.sql("SELECT UrlDecoder(<YOUR_COL>) from <YOUR_TABLE>;")
+   ```

tests/udf/src/com/intel/oap/URLDecoderNew.java

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+/*
+ * 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 com.intel.oap;
+
+import java.net.URLDecoder;
+import org.apache.hadoop.hive.ql.exec.UDF;
+
+public class URLDecoderNew extends UDF {
+
+  public String evaluate(String b) {
+    if (b == null) {
+      return null;
+    }
+    try {
+      return URLDecoder.decode(b, "utf-8");
+    } catch (Exception e) {
+      e.printStackTrace();
+      return b;
+    }
+  }
+}