- [x] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [x] I have checked the [FAQ](https://github.com/apache/dubbo/blob/master/FAQ.md) of this repository and believe that this is not a duplicate.

### Environment

* Dubbo version: 3.0.0


### Steps to reproduce this issue

1. dubbo config-center 相关yml配置如下
```
dubbo:
  ...
  config-center:
    address: nacos://xxxx:8848
    username: someusername
    password: xxxx
```

2. 在执行 org.apache.dubbo.config.bootstrap.DubboBootstrap#startConfigCenter 方法时会调用 configCenterConfig.refresh(); 方法。
3. setAddress方法，会覆盖已经设置过的username和password

```
public void setAddress(String address) {
        this.address = address;
        if (address != null) {
            try {
                URL url = URL.valueOf(address);
                setUsername(url.getUsername());
                setPassword(url.getPassword());
                updateIdIfAbsent(url.getProtocol());
                updateProtocolIfAbsent(url.getProtocol());
                updatePortIfAbsent(url.getPort());
                updateParameters(url.getParameters());
            } catch (Exception ignored) {
            }
        }
    }
```

### Expected Result

希望设置前先判断是否已经设置过相关属性值



/* The group of the config center, generally it's used to identify an isolated space for a batch of config items,
    but it's real meaning depends on the actual Config Center you use.
    */
    private String group; 
    private String username;
    private String password;
    private Long timeout; 
    /**
     * If the Config Center is given the highest priority, it will override all the other configurations
     * @deprecated no longer used
     */
    private Boolean highestPriority; 
    
    private Boolean check; 
    /* Used to specify the key that your properties file mapping to, most of the time you do not need to change this parameter.
    Notice that for Apollo, this parameter is meaningless, set the 'namespace' is enough.
    */
    private String configFile; 
    /* the .properties file under 'configFile' is global shared while .properties under this one is limited only to this application
    */
    private String appConfigFile;
    /* If the Config Center product you use have some special parameters that is not covered by this class, you can add it to here.
    For example, with XML:
      <dubbo:config-center>
           <dubbo:parameter key="{your key}" value="{your value}" />
      </dubbo:config-center>
     */
    private Map<String, String> parameters;
    private Map<String, String> externalConfiguration;
    private Map<String, String> appExternalConfiguration;