[composer][6/7]Implement LRO rpc callable method's sample code

[summer-ji-eng]

There are two kind sample code for lro callable:
1.

   * Sample code:
   *
   * <pre>
   * <code>
   * try (EchoClient echoClient = EchoClient.create()) {
   *   WaitRequest request = WaitRequest.newBuilder().build();
   *   OperationFuture&lt;WaitResponse, WaitMetadata&gt; future = echoClient.waitOperationCallable().futureCall(request);
   *   // Do something
   *   WaitResponse response = future.get();
   * }
   * </code>
   * </pre>
   */
  @BetaApi("The surface for use by generated code is not stable yet and may change in the future.")
  public final OperationCallable<WaitRequest, WaitResponse, WaitMetadata> waitOperationCallable() {
    return stub.waitOperationCallable();
  }

   * Sample code:
   *
   * <pre>
   * <code>
   * try (EchoClient echoClient = EchoClient.create()) {
   *   WaitRequest request = WaitRequest.newBuilder().build();
   *   ApiFuture&lt;Operation&gt; future = echoClient.waitCallable().futureCall(request);
   *   // Do something
   *   Operation response = future.get();
   * }
   * </code>
   * </pre>
   */
  public final UnaryCallable<WaitRequest, Operation> waitCallable() {
    return stub.waitCallable();
  }

[summer-ji-eng]

refactor #637