Service Discovery & Configuration through xDS

# 题目3 - Service Discovery & Configuration through xDS

支持基于 xDS 的服务发现、服务配置，使得 Dubbo 可直接与 Mesh 架构的控制面通信

## 背景

xDS 是 Mesh 架构下定义的一套服务配置下发的通用协议，最开始来自于 Envoy，用于实现控制面与数据面的数据/配置下发。xDS 是一系列配置 API 的统称：

- **Listener Discovery Service (LDS)**: Returns [`Listener`](https://github.com/envoyproxy/envoy/blob/9e83625b16851cdc7e4b0a4483b0ce07c33ba76b/api/envoy/api/v2/listener.proto#L27) resources. Used basically as a convenient root for the gRPC client's configuration. Points to the RouteConfiguration.
- **Route Discovery Service (RDS)**: Returns [`RouteConfiguration`](https://github.com/envoyproxy/envoy/blob/9e83625b16851cdc7e4b0a4483b0ce07c33ba76b/api/envoy/api/v2/route.proto#L24) resources. Provides data used to populate the gRPC [service config](https://github.com/grpc/grpc/blob/master/doc/service_config.md). Points to the Cluster.
- **Cluster Discovery Service (CDS)**: Returns [`Cluster`](https://github.com/envoyproxy/envoy/blob/9e83625b16851cdc7e4b0a4483b0ce07c33ba76b/api/envoy/api/v2/cluster.proto#L34) resources. Configures things like load balancing policy and load reporting. Points to the ClusterLoadAssignment.
- **Endpoint Discovery Service (EDS)**: Returns [`ClusterLoadAssignment`](https://github.com/envoyproxy/envoy/blob/9e83625b16851cdc7e4b0a4483b0ce07c33ba76b/api/envoy/api/v2/endpoint.proto#L33) resources. Configures the set of endpoints (backend servers) to load balance across and may tell the client to drop requests.

## 实现方案（目标）

主要是 Client 端对接 xDS 协议

* 实现基于 xDS 的服务发现
* 实现基于 xDS 的配置和规则下发

可参加：https://github.com/markdroth/proposal/blob/34dee185a28e82b0ac422f4642f6b81862d62e36/A27-xds-global-load-balancing.md

No repository files, tests, or implementation entry points are named. Start by reading the xDS LDS, RDS, CDS, and EDS descriptions and the linked A27 proposal, then locate Dubbo's client-side discovery and configuration components. Done means the client supports xDS-based service discovery plus delivery of configuration and routing rules from a Mesh control plane.

Written by the indexing model from the issue text.