[Proposal] Add Dubbo cluster extension for OpenSergo, support traffic routing capability.

[panxiaojun233]

OpenSergo is an open, language-agnostic cloud-native service governance specification that is close to business semantics. OpenSergo Control Plane is the control component of OpenSergo CRD, carries the responsibility of service governance configuration conversion and distribution. OpenSergo Control Plane defined the TrafficRouter rules, which based on Istio VirtualService .

Hope Dubbo can support OpenSergo TrafficRouter, through Sentinel2.0's Traffic Routing Capabilities Quickly support traffic routing capabilitiy which following OpenSergo service governance practices.

OpenSergo 是开放通用的，覆盖微服务及上下游关联组件的微服务治理项目，其中 OpenSergo 控制平面 (Control Plane) 作为 OpenSergo CRD 的统一管控组件，承载服务治理配置转换与下发的职责。OpenSergo 控制面定义了特定的流量路由规则 TrafficRouter。TrafficRouter 将特定特征的流量映射至特定特征所对应的工作负载上，社区目前的设计TrafficRouter主要是基于 Istio VirtualService 进行补充与扩展。

希望 Dubbo 可以支持 OpenSergo 流量路由规则 TrafficRouter ，通过对接 Sentinel2.0 的流量路由能力 快速实现遵循 OpenSergo 服务治理实践的流量路由能力。

[sczyh30]

Relevant PR: apache/dubbo-spi-extensions#192