Generate documentation of PulsarProxy automatically (apache#11421)

Master Issue: apache#10041

### Motivation
The config object has already been annotated.
We just need to add it to the auto-generated list

pulsar-proxy/src/main/java/org/apache/pulsar/proxy/util/CmdGenerateDocumentation.java

@@ -0,0 +1,45 @@
+/**
+ * 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.pulsar.proxy.util;
+
+import com.beust.jcommander.Parameters;
+import lombok.Data;
+import lombok.extern.slf4j.Slf4j;
+import org.apache.pulsar.broker.BaseGenerateDocumentation;
+import org.apache.pulsar.proxy.server.ProxyConfiguration;
+
+@Data
+@Parameters(commandDescription = "Generate documentation automatically.")
+@Slf4j
+public class CmdGenerateDocumentation extends BaseGenerateDocumentation {
+
+    @Override
+    public String generateDocumentByClassName(String className) throws Exception {
+        StringBuilder sb = new StringBuilder();
+        if (ProxyConfiguration.class.getName().equals(className)) {
+            return generateDocByFieldContext(className, "Pulsar proxy", sb);
+        }
+        return "Class [" + className + "] not found";
+    }
+
+    public static void main(String[] args) throws Exception {
+        CmdGenerateDocumentation generateDocumentation = new CmdGenerateDocumentation();
+        generateDocumentation.run(args);
+    }
+}

pulsar-proxy/src/test/java/org/apache/pulsar/proxy/util/CmdTest.java

@@ -0,0 +1,58 @@
+/**
+ * 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.pulsar.proxy.util;
+
+import static org.testng.Assert.assertTrue;
+import java.io.ByteArrayOutputStream;
+import java.io.PrintStream;
+import java.lang.reflect.Field;
+import org.apache.pulsar.common.configuration.FieldContext;
+import org.testng.annotations.Test;
+
+public class CmdTest {
+
+    @Test
+    public void cmdParserProxyConfigurationTest() throws Exception {
+        String value = generateDoc("org.apache.pulsar.proxy.server.ProxyConfiguration");
+        assertTrue(value.contains("Pulsar proxy"));
+    }
+
+    private String generateDoc(String clazz) throws Exception {
+        PrintStream oldStream = System.out;
+        try (ByteArrayOutputStream baoStream = new ByteArrayOutputStream(2048);
+             PrintStream cacheStream = new PrintStream(baoStream);) {
+            System.setOut(cacheStream);
+            CmdGenerateDocumentation.main(("-c " + clazz).split(" "));
+            String message = baoStream.toString();
+            Class cls = Class.forName(clazz);
+            Field[] fields = cls.getDeclaredFields();
+            for (Field field : fields) {
+                field.setAccessible(true);
+                FieldContext fieldContext = field.getAnnotation(FieldContext.class);
+                if (fieldContext == null) {
+                    continue;
+                }
+                assertTrue(message.indexOf(field.getName()) > 0);
+            }
+            return message;
+        } finally {
+            System.setOut(oldStream);
+        }
+    }
+}