[transaction] Add transaction coordinator client (apache#4953)

*Motivation*

Add the interface of transaction coordinator client. So that it can be used
by coordinator for committing transactions on topics and subscriptions.

NOTE: this PR is only introducing the interface. The implementation will come
in subsequent pull requests

pulsar-client/src/main/java/org/apache/pulsar/client/impl/transaction/TransactionCoordinatorClient.java

@@ -0,0 +1,80 @@
+/**
+ * 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.client.impl.transaction;
+
+import java.util.concurrent.CompletableFuture;
+
+/**
+ * The transaction coordinator client to commit and abort transactions on topics.
+ */
+public interface TransactionCoordinatorClient {
+
+    /**
+     * Commit the transaction associated with the topic.
+     *
+     * @param topic topic name
+     * @param txnIdMostBits the most bits of txn id
+     * @param txnIdLeastBits the least bits of txn id
+     * @return the future represents the commit result
+     */
+    CompletableFuture<Void> commitTxnOnTopic(String topic,
+                                             long txnIdMostBits,
+                                             long txnIdLeastBits);
+
+    /**
+     * Abort the transaction associated with the topic.
+     *
+     * @param topic topic name
+     * @param txnIdMostBits the most bits of txn id
+     * @param txnIdLeastBits the least bits of txn id
+     * @return the future represents the abort result
+     */
+    CompletableFuture<Void> abortTxnOnTopic(String topic,
+                                            long txnIdMostBits,
+                                            long txnIdLeastBits);
+
+    /**
+     * Commit the transaction associated with the topic subscription.
+     *
+     * @param topic topic name
+     * @param subscription subscription name
+     * @param txnIdMostBits the most bits of txn id
+     * @param txnIdLeastBits the least bits of txn id
+     * @return the future represents the commit result
+     */
+    CompletableFuture<Void> commitTxnOnSubscription(String topic,
+                                                    String subscription,
+                                                    long txnIdMostBits,
+                                                    long txnIdLeastBits);
+
+    /**
+     * Abort the transaction associated with the topic subscription.
+     *
+     * @param topic topic name
+     * @param subscription subscription name
+     * @param txnIdMostBits the most bits of txn id
+     * @param txnIdLeastBits the least bits of txn id
+     * @return the future represents the abort result
+     */
+    CompletableFuture<Void> abortTxnOnSubscription(String topic,
+                                                   String subscription,
+                                                   long txnIdMostBits,
+                                                   long txnIdLeastBits);
+
+}

pulsar-client/src/main/java/org/apache/pulsar/client/impl/transaction/TransactionCoordinatorClientImpl.java

@@ -0,0 +1,59 @@
+/**
+ * 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.client.impl.transaction;
+
+import java.util.concurrent.CompletableFuture;
+import org.apache.pulsar.client.impl.PulsarClientImpl;
+import org.apache.pulsar.common.util.FutureUtil;
+
+/**
+ * The implementation of {@link TransactionCoordinatorClient}.
+ */
+public class TransactionCoordinatorClientImpl implements TransactionCoordinatorClient {
+
+    private final PulsarClientImpl client;
+
+    public TransactionCoordinatorClientImpl(PulsarClientImpl client) {
+        this.client = client;
+    }
+
+    @Override
+    public CompletableFuture<Void> commitTxnOnTopic(String topic, long txnIdMostBits, long txnIdLeastBits) {
+        return FutureUtil.failedFuture(
+            new UnsupportedOperationException("Not Implemented Yet"));
+    }
+
+    @Override
+    public CompletableFuture<Void> abortTxnOnTopic(String topic, long txnIdMostBits, long txnIdLeastBits) {
+        return FutureUtil.failedFuture(
+            new UnsupportedOperationException("Not Implemented Yet"));
+    }
+
+    @Override
+    public CompletableFuture<Void> commitTxnOnSubscription(String topic, String subscription, long txnIdMostBits, long txnIdLeastBits) {
+        return FutureUtil.failedFuture(
+            new UnsupportedOperationException("Not Implemented Yet"));
+    }
+
+    @Override
+    public CompletableFuture<Void> abortTxnOnSubscription(String topic, String subscription, long txnIdMostBits, long txnIdLeastBits) {
+        return FutureUtil.failedFuture(
+            new UnsupportedOperationException("Not Implemented Yet"));
+    }
+}

pulsar-client/src/main/java/org/apache/pulsar/client/impl/transaction/package-info.java

@@ -0,0 +1,22 @@
+/**
+ * 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.
+ */
+/**
+ * Transaction client implementation.
+ */
+package org.apache.pulsar.client.impl.transaction;