Support Gray Released (Migration scenario)

- [ ] I have searched the [issues](https://github.com/apache/dubbo/issues) of this repository and believe that this is not a duplicate.
- [ ] I have searched the [release notes](https://github.com/apache/dubbo/releases) of this repository and believe that this is not a duplicate.

# Background

I am currently doing a service migration, which will generate multiple services and I need to test the availability of the service.

# Describe

Therefore, it is desirable to have a way to dynamically route to old and new services.Roll back when problems occur with the new service.

For example, access to the configuration center, adding a switch to control routing to new and old services.

Further, whether can increase gray scale proportion.

> This function is similar to gray released

Start by reviewing the existing gray-release, service-routing, and configuration-center entry points in Apache Dubbo. Clarify how migration traffic should switch between old and new services, how rollback should work, and how gray-scale proportions should be controlled before defining implementation and tests.

Written by the indexing model from the issue text.