[FEATURE] etcdv3 registry: implement DoUnregister, DoUnsubscribe and reconnect subscription recovery

[XnLemon]

⚠️ Verification

• I have searched the issues of this repository and believe that this is not a duplicate.
• I have searched the release notes of this repository and believe that this is not a duplicate.

🎯 Solution Description

The etcdv3 registry had three critical gaps compared to the zookeeper registry:

1. DoUnregister was not implemented — always returned "DoUnregister is not support in etcdV3Registry".
   The fix deletes the corresponding etcd key via client.Delete, guarded by a client.Valid() check.

2. DoUnsubscribe was not implemented — always returned an error.
   The fix introduces a per-service listener architecture: dataListener now holds a
   subscribed map[string]ConfigurationListener keyed by service key, replacing the old
   single shared configurationListener. Each DoSubscribe call creates an independent
   configurationListener for that service; DoUnsubscribe closes and removes it.

3. Subscriptions were silently lost after reconnect — InitListeners (called by RestartCallBack
   on reconnect) replaced dataListener with an empty new instance, discarding all active
   subscriptions. The fix stores subscribeURL *common.URL on each configurationListener and
   adds a recovery loop in InitListeners that mirrors the zookeeper registry: close stale
   listeners, create fresh ones, restart ListenServiceEvent goroutines for every recovered URL.

etcdv3 注册中心相比 zookeeper 注册中心存在三个关键缺口：

1. DoUnregister 未实现 — 始终返回 "DoUnregister is not support in etcdV3Registry"。
   修复：通过 client.Delete 删除对应 etcd key，调用前做 client.Valid() 校验。

2. DoUnsubscribe 未实现 — 始终返回错误。
   修复：引入 per-service listener 架构，dataListener 改为以 service key 为键的
   subscribed map，取代原来全局共享的单个 configurationListener。每次 DoSubscribe
   为该服务创建独立的 configurationListener；DoUnsubscribe 关闭并移除对应 listener。

3. 断线重连后订阅静默丢失 — InitListeners（重连时由 RestartCallBack 触发）将
   dataListener 替换为空实例，所有活跃订阅被丢弃。
   修复：在 configurationListener 上存储 subscribeURL *common.URL，在 InitListeners
   中加入与 zookeeper 注册中心对齐的恢复循环：关闭旧 listener，为每个已订阅 URL 创建新
   listener，重新启动 ListenServiceEvent goroutine。

📋 Use Cases

• Services that need to deregister from etcd on graceful shutdown.

• Services that dynamically subscribe/unsubscribe to providers at runtime.

• Long-running applications deployed in environments with unstable etcd connections (container
  restarts, network partitions, etcd leader elections) that require seamless reconnect without
  losing active subscriptions.

• 需要在优雅停机时从 etcd 注销的服务。

• 运行时动态订阅/取消订阅 provider 的服务。

• 部署在网络不稳定环境（容器重启、网络分区、etcd leader 选举）中、要求无感知重连且不丢失
  订阅的长期运行应用。

⚖️ Complexity & Risks

• Medium complexity. The per-service listener refactor touches the core subscribe/unsubscribe path.

• DataChange now iterates all subscribed keys (instead of returning on first match), which is
  a correctness improvement for wildcard/any-condition scenarios but changes the semantics slightly.

• InitListeners holds the old dataListener mutex for the duration of recovery via defer,
  which may briefly delay concurrent DataChange calls — same trade-off as zookeeper.

• 中等复杂度，per-service listener 重构影响核心订阅/取消订阅路径。

• DataChange 改为遍历全部匹配 key（原先匹配即返回），对 wildcard/any-condition 场景是正确性
  修复，但语义有轻微变化。

• InitListeners 通过 defer 持有旧 dataListener mutex 直至恢复完成，大量订阅时可能短暂
  阻塞并发 DataChange，与 zookeeper 注册中心权衡一致。

🔗 External Dependencies

None. Changes are confined to registry/etcdv3/listener.go and registry/etcdv3/registry.go,
using existing remoting/etcdv3 and common.URL APIs.

无外部依赖，改动限于 registry/etcdv3/listener.go 和 registry/etcdv3/registry.go，
使用已有的 remoting/etcdv3 和 common.URL API。

📘 Additional Context

Reference implementation for reconnect recovery: registry/zookeeper/registry.go — InitListeners (line 92-120).

重连恢复逻辑参考实现：registry/zookeeper/registry.go — InitListeners（第 92-120 行）。

如果觉得可以的话Assign me 然后我了解到我们目前需要将新注册中心迁移到dubbo-go-extension内作为插件使用 可以实现后进行迁移

[Alanxtl]

确实适合迁移到 dubbo-go-extension
不过 etcdv3 的活不用着急做 如果对其他任务感兴趣的话可以先做其他的