private String getString(String key, String defaultValue) {
        return get(key, Function.identity(), defaultValue);
    }
the properties of polaris registry

@author <a href=mailto:bner666@gmail.com>ZhangLibin</a>
registry/registry-polaris/src/main/java/com/alipay/sofa/rpc/registry/polaris/PolarisRegistryProperties.java