complete task 4 in #5923 , modify dubbo registry config base on selector.

1. i add one parameter "registry" in dubbo selector, and change "protocol" to not required.
   
2. for example, change the shenyu-examples-apache-dubbo-service's dubbo.registry.address to 'nacos://localhost:8848?namespace=local_test&username=nacos&password=nacos'
   
   if the dubbo selector parameter registry is empty, it will use the default plugin config. i add a common method to get the dubbo ReferenceConfig in ApacheDubboProxyService.java#getReferenceConfig
   
   when invoke the api 'http://localhost:9195/dubbo/findById', it will throw No provider exception.
   
3. set the dubbo selector parameter to 'nacos://localhost:8848?namespace=local_test&username=nacos&password=nacos'
   
   when invoke the api the 'dubboUpstreams' is not empty in ApacheDubboProxyService.java#getReferenceConfig
   
   then chose one upstream and create a new ReferenceConfig with a new cacheKey
   
   finally, it will use the new ReferenceConfig to invoke the dubbo interface through the new registry。
   
4. when build the ReferenceConfig in ApacheDubboConfigCache.java, it will use the new dubbo RegistryConfig and the dubbo rule config is not used in default model. so i wrote a new buildReference method。
   
   And when use the multi registry or there are multiple Dubbo instances, the custom dubbo LoadBalance ApacheDubboGrayLoadBalance will be triggered, so when build the ReferenceConfig i didn't use reference.setLoadbalance("gray") and methodConfig.setLoadbalance("gray").

Make sure that:

• You have read the contribution guidelines.
• You submit test cases (unit or integration tests) that back your changes.
• Your local test passed ./mvnw clean install -Dmaven.javadoc.skip=true.