Fix issue 1317

[chuntaojun]

Please do not create a Pull Request without creating an issue first.

thanks @paderlol and @HaojunRen help

What is the purpose of the change

fix issue #1317

Add a listener when the configuration is obtained

Brief changelog

There are usage scenarios as follows: When the program starts, the user registers the Listener and uses getConfig to obtain the configuration information of the remote end, but does not want to trigger the Listener. Instead, after the getConfig, if the configuration information changes, the Listener is triggered.

Verifying this change

 @Test
public void nacos_addListener_5() throws InterruptedException, NacosException {
    final AtomicInteger count = new AtomicInteger(0);
    final String content = "test-abc";
    final String newContent = "new-test-def";
    boolean result = iconfig.publishConfig(dataId, group, content);
    Assert.assertTrue(result);

    Thread.sleep(3000);

    Listener ml = new AbstractListener() {
        @Override
        public void receiveConfigInfo(String configInfo) {
            count.incrementAndGet();
            System.out.println("Listener receive : [" + configInfo + "]");
            Assert.assertEquals(content, newContent);
        }
    };
    String receiveContent = iconfig.getConfigAndSignListener(dataId, group, 1000, ml);
    System.out.println(receiveContent);

    result = iconfig.publishConfig(dataId, group, newContent);
    Assert.assertTrue(result);

    Assert.assertEquals(content, receiveContent);
    Thread.sleep(3000);

    Assert.assertEquals(1, count.get());
    iconfig.removeListener(dataId, group, ml);
}

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